org.texi: Document `org-agenda-search-headline-for-time'

* doc/org.texi (Time-of-day specifications): Document
  `org-agenda-search-headline-for-time'.

Reported-by: Rohit Patnaik <quanticle@quanticle.net>
<http://lists.gnu.org/r/emacs-orgmode/2018-04/msg00405.html>

doc/org.texi

@@ -8663,9 +8663,13 @@ ranges can be specified with two timestamps, like
 @c
 @w{@samp{<2005-05-10 Tue 20:30>--<2005-05-10 Tue 22:15>}}.
 
-In the headline of the entry itself, a time(range) may also appear as
-plain text (like @samp{12:45} or a @samp{8:30-1pm}).  If the agenda
-integrates the Emacs diary (@pxref{Weekly/daily agenda}), time
+@vindex org-agenda-search-headline-for-time
+In the headline of the entry itself, a time(range) may also appear as plain
+text (like @samp{12:45} or a @samp{8:30-1pm})@footnote{You can, however,
+disable this by setting @code{org-agenda-search-headline-for-time} variable
+to a @code{nil} value.}.
+
+If the agenda integrates the Emacs diary (@pxref{Weekly/daily agenda}), time
 specifications in diary entries are recognized as well.
 
 For agenda display, Org mode extracts the time and displays it in a