Add note about the info documentation of diff(1).

It contains a detailed description of the =format option part.

PR:		docs/36464
Submitted by:	Gary W. Swearingen <swear@blarg.net>

gnu/usr.bin/diff/diff.1

@@ -141,6 +141,10 @@ Use
 .I format
 to output a line group containing differing lines from
 both files in if-then-else format.
+For more details on what can be specified in the
+.I format
+string, consult the info documentation of
+.I diff .
 .TP
 .B \-d
 Change the algorithm to perhaps find a smaller set of changes.  This makes