org,org-agenda: New: open info depending on context

* lisp/org.el (org-info-find-node): Function to jump to info depending
  on context.  For an org file the context is the element type at point.

doc/org-manual.org

@@ -18962,6 +18962,22 @@ further based on their usage needs.  For example, the normal
 | {{{kbd(C-S-LEFT)}}}  | {{{kbd(C-c C-x LEFT)}}}  |              |                      |
 | {{{kbd(C-S-RIGHT)}}} | {{{kbd(C-c C-x RIGHT)}}} |              |                      |
 
+** Context Dependent Documentation
+:PROPERTIES:
+:DESCRIPTION: Get Documentation for Context.
+:ALT_TITLE: Documentation
+:END:
+#+cindex: documentation
+#+cindex: Info
+#+findex: org-info-find-node
+
+{{{kbd(C-c C-x C-i)}}} tries to open a suitable section of the Org
+info documentation depending on the Org element at point.  For example
+on a headline the info documentation about the Org document structure
+appears.
+
+{{{kbd(q)}}} closes the info window.
+
 ** Interaction with Other Packages
 :PROPERTIES:
 :DESCRIPTION: With other Emacs packages.

etc/ORG-NEWS

@@ -91,7 +91,7 @@ changed.  This is an excerpt of the new docstring:
 : When set to ‘default’, bind the function to ‘c’, but only if it is
 : available in the Calendar keymap.  This is the default choice because
 : ‘c’ can then be used to switch back and forth between agenda and calendar.
-: 
+:
 : When nil, ‘org-calendar-goto-agenda’ is not bound to any key.
 
 Check the full docstring for more.
@@ -101,7 +101,7 @@ Check the full docstring for more.
 Here is the new docstring:
 
 : (org-set-effort &optional INCREMENT VALUE)
-: 
+:
 : Set the effort property of the current entry.
 : If INCREMENT is non-nil, set the property to the next allowed
 : value.  Otherwise, if optional argument VALUE is provided, use
@@ -345,6 +345,13 @@ about the last major release.
 
 There is a new menu entry for this in the "Documentation" menu item.
 
+*** ~org-info-find-node~
+
+From an Org file or an agenda switch to a suitable info page depending
+on the context.
+
+The function is bound to =C-c C-x I=.
+
 ** Removed commands and functions
 *** ~org-outline-overlay-data~
 Use ~org-save-outline-visibility~ instead.

lisp/org-agenda.el

@@ -2395,6 +2395,7 @@ The following commands are available:
 (define-key org-agenda-mode-map "?" 'org-agenda-show-the-flagging-note)
 (org-defkey org-agenda-mode-map "\C-c\C-x\C-mg"    'org-mobile-pull)
 (org-defkey org-agenda-mode-map "\C-c\C-x\C-mp"    'org-mobile-push)
+(org-defkey org-agenda-mode-map "\C-c\C-xI" 'org-info-find-node)
 
 (org-defkey org-agenda-mode-map [mouse-2] 'org-agenda-goto-mouse)
 (org-defkey org-agenda-mode-map [mouse-3] 'org-agenda-show-mouse)

lisp/org.el

@@ -19105,6 +19105,7 @@ COMMANDS is a list of alternating OLDDEF NEWDEF command names."
 (org-defkey org-mode-map (kbd "C-c C-x _") #'org-timer-stop)
 (org-defkey org-mode-map (kbd "C-c C-x ;") #'org-timer-set-timer)
 (org-defkey org-mode-map (kbd "C-c C-x ,") #'org-timer-pause-or-continue)
+(org-defkey org-mode-map (kbd "C-c C-x I") #'org-info-find-node)
 
 (define-key org-mode-map (kbd "C-c C-x C-c") #'org-columns)
 
@@ -23661,6 +23662,57 @@ when non-nil, is a regexp matching keywords names."
 	  "\\):[ \t]*\\(.*\\)"))
 
 
+;;; Conveniently switch to Info nodes
+
+(defun org-info-find-node (&optional nodename)
+  "Find Info documentation NODENAME or Org documentation according context.
+Started from `gnus-info-find-node'."
+  (interactive)
+  (Info-goto-node
+   (or nodename
+       (let ((default-org-info-node "(org) Top"))
+         (cond
+          ((eq 'org-agenda-mode major-mode) "(org) Agenda Views")
+          ((eq 'org-mode major-mode)
+           (let* ((context (org-element-at-point))
+                  (element-info-nodes ; compare to `org-element-all-elements'.
+                   `(
+                     (babel-call . "(org) Evaluating Code Blocks")
+                     (center-block . "(org) Paragraphs")
+                     (clock . ,default-org-info-node)
+                     (comment . "(org) Comment Lines")
+                     (comment-block . "(org) Comment Lines")
+                     (diary-sexp . ,default-org-info-node)
+                     (drawer . "(org) Drawers")
+                     (dynamic-block . "(org) Dynamic Blocks")
+                     (example-block . "(org) Literal Examples")
+                     (export-block . "(org) ASCII/Latin-1/UTF-8 export")
+                     (fixed-width . ,default-org-info-node)
+                     (footnote-definition . "(org) Creating Footnotes")
+                     (headline . "(org) Document Structure")
+                     (horizontal-rule . "(org) Built-in Table Editor")
+                     (inlinetask . ,default-org-info-node)
+                     (item . "(org) Plain Lists")
+                     (keyword . "(org) Per-file keywords")
+                     (latex-environment . "(org) LaTeX Export")
+                     (node-property . "(org) Properties and Columns")
+                     (paragraph . "(org) Paragraphs")
+                     (plain-list . "(org) Plain Lists")
+                     (planning . "(org) Deadlines and Scheduling")
+                     (property-drawer . "(org) Properties and Columns")
+                     (quote-block . "(org) Paragraphs")
+                     (section . ,default-org-info-node)
+                     (special-block . ,default-org-info-node)
+                     (src-block . "(org) Working with Source Code")
+                     (table . "(org) Tables")
+                     (table-row . "(org) Tables")
+                     (verse-block . "(org) Paragraphs")
+                     )))
+             (or (cdr (assoc (car context) element-info-nodes))
+                 default-org-info-node)))
+          (t default-org-info-node))))))
+
+
 ;;; Finish up
 
 (add-hook 'org-mode-hook     ;remove overlays when changing major mode