Document that 'git -C ""' works and doesn't change directory

It's been behaving so since 6a536e2076 (git: treat "git -C '<path>'"
as a no-op when <path> is empty, 2015-03-06).

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -56,7 +56,8 @@ help ...`.
 	Run as if git was started in '<path>' instead of the current working
 	directory.  When multiple `-C` options are given, each subsequent
 	non-absolute `-C <path>` is interpreted relative to the preceding `-C
-	<path>`.
+	<path>`.  If '<path>' is present but empty, e.g. `-C ""`, then the
+	current working directory is left unchanged.
 +
 This option affects options that expect path name like `--git-dir` and
 `--work-tree` in that their interpretations of the path names would be