Documentation/git-log: mention that line-log regex must match in starting revision

When giving a regex as parameter <start> or <end> in
`git log -L <start>,<end>:<file>`, or a function name in
`git log -L :<funcname>:<file>`, the given regex must match in the starting
revision, or else the command exits with a fatal error.

This is not obvious in the documentation, so add a note to that
effect.

Signed-off-by: Philippe Blain <[email protected]>

Author: phil-blain

Documentation/git-log.txt

@@ -76,7 +76,8 @@ produced by `--stat`, etc.
 	(or the function name regex <funcname>) within the <file>.  You may
 	not give any pathspec limiters.  This is currently limited to
 	a walk starting from a single revision, i.e., you may only
-	give zero or one positive revision arguments.
+	give zero or one positive revision arguments, and any given regex for
+	<start> or <end> (or <funcname>) must match in the starting revision.
 	You can specify this option more than once. Implies `--patch`.
 	If ``:<funcname>'' is given, implies `--function-context`.
 	Patch output can be suppressed using `-s`, but other diff formats