Documentation: update description of shell aliases

Aliases that invoke shell commands start from the top-level directory,
but this was not documented.

Signed-off-by: Sitaram Chamarty <sitaramc@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -451,7 +451,9 @@ If the alias expansion is prefixed with an exclamation point,
 it will be treated as a shell command.  For example, defining
 "alias.new = !gitk --all --not ORIG_HEAD", the invocation
 "git new" is equivalent to running the shell command
-"gitk --all --not ORIG_HEAD".
+"gitk --all --not ORIG_HEAD".  Note that shell commands will be
+executed from the top-level directory of a repository, which may
+not necessarily be the current directory.
 
 apply.whitespace::
 	Tells 'git-apply' how to handle whitespaces, in the same way