Doc: shallow clone deepens _to_ new depth

Clarify that 'depth=' specifies the new depth from the remote's
branch tip. It does not add the depth to the existing shallow clone.
(details from pack-protocol.txt).
Clarify that tags are not fetched. (details from shallow.txt)

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/fetch-options.txt

@@ -10,7 +10,8 @@
 --depth=<depth>::
 	Deepen the history of a 'shallow' repository created by
 	`git clone` with `--depth=<depth>` option (see linkgit:git-clone[1])
-	by the specified number of commits.
+	to the specified number of commits from the tip of each remote
+	branch history. Tags for the deepened commits are not fetched.
 
 ifndef::git-pull[]
 --dry-run::