git-clean doc: the command only affects paths under $(cwd)

Fredrik Skolmli and Thomas Rast noticed that it was left unstated that
"git clean" ran from a subdirectory will not affect anything outside it,
with or without path limiters.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clean.txt

@@ -13,8 +13,8 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
-This allows cleaning the working tree by removing files that are not
-under version control.
+Cleans the working tree by recursively removing files that are not
+under version control, starting from the current directory.
 
 Normally, only files unknown to git are removed, but if the '-x'
 option is specified, ignored files are also removed. This can, for