git-fetch.txt: improve description of tag auto-following

Make it clearer that tags are fetched independent of which branches
were fetched from the remote in any particular fetch.  (Tags are even
fetched if they point at objects that are in the current repository
but not reachable, which is probably a bug.)

Put less emphasis on the mechanism and more on the effect of tag
auto-following.  Also mention the options and configuration settings
that can change the tag-fetching behavior.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fetch.txt

@@ -24,13 +24,13 @@ The ref names and their object names of fetched refs are stored
 in `.git/FETCH_HEAD`.  This information is left for a later merge
 operation done by 'git merge'.
 
-When <refspec> stores the fetched result in remote-tracking branches,
+By default, tags are auto-followed.  This means that when fetching
-the tags that point at these branches are automatically
+from a remote, any tags on the remote that point to objects that exist
-followed.  This is done by first fetching from the remote using
+in the local repository are fetched.  The effect is to fetch tags that
-the given <refspec>s, and if the repository has objects that are
+point at branches that you are interested in.  This default behavior
-pointed by remote tags that it does not yet have, then fetch
+can be changed by using the --tags or --no-tags options, by
-those missing tags.  If the other end has tags that point at
+configuring remote.<name>.tagopt, or by using a refspec that fetches
-branches you are not interested in, you will not get them.
+tags explicitly.
 
 'git fetch' can fetch from either a single named repository,
 or from several repositories at once if <group> is given and