don't mention index refreshing side effect in git-status docs

The tip about speeding up subsequent operations is now
obsolete; since aecbf91, git-diff now squelches empty diffs
and performs an automatic refresh.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-status.txt

@@ -28,13 +28,6 @@ If there is no path that is different between the index file and
 the current HEAD commit (i.e., there is nothing to commit by running
 `git-commit`), the command exits with non-zero status.
 
-If any paths have been touched in the working tree (that is,
-their modification times have changed) but their contents and
-permissions are identical to those in the index file, the command
-updates the index file. Running `git-status` can thus speed up
-subsequent operations such as `git-diff` if the working tree
-contains many paths that have been touched but not modified.
-
 
 OUTPUT
 ------