fetch: elaborate --depth action

--depth is explained as deepen, but the way it's applied, it can
shorten the history as well. Keen users may have noticed the
implication by the phrase "the specified number of commits from the
tip of each remote branch". Put "shorten" in the description to make
it clearer.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/fetch-options.txt

@@ -8,7 +8,7 @@
 	option old data in `.git/FETCH_HEAD` will be overwritten.
 
 --depth=<depth>::
-	Deepen the history of a 'shallow' repository created by
+	Deepen or shorten the history of a 'shallow' repository created by
 	`git clone` with `--depth=<depth>` option (see linkgit:git-clone[1])
 	to the specified number of commits from the tip of each remote
 	branch history. Tags for the deepened commits are not fetched.