Add a cross ref to Optional Mode Line

* doc/lispref/modes.texi (Mode Line Variables): Add a cross
reference to the Emacs mode line node that explains things
like `display-time-string' (bug#21002).

(cherry picked from commit a3151a28789f413af73b14fbba557b2a587fca53)

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 52cc8f86bcf5b8281657cc537f4d3c1ba18f5b6e..d2eebb7ddb442087a9093989343335005e0d7b73 100644 (file)
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -1912,7 +1912,8 @@ could have the same effects on the mode line if the value of
 @code{mode-line-format} is changed to use them.  However, various parts
 of Emacs set these variables on the understanding that they will control
 parts of the mode line; therefore, practically speaking, it is essential
-for the mode line to use them.
+for the mode line to use them.  Also see
+@ref{Optional Mode Line,,, emacs, The GNU Emacs Manual}.
 
 @defvar mode-line-mule-info
 This variable holds the value of the mode line construct that displays