am doc: add a pointer to relevant hooks

It is not obvious when looking at a new command what hooks will affect
it.  Add a HOOKS section to the git-am(1) page, imitating
git-commit(1), to make it easier for people to discover e.g. the
applypatch-msg hook that can implement a custom subject-mangling
strategy (e.g., removing a "bug #nnnn:" prefix introduced by a bug
tracker).

Reported-by: Phillip Susi <[email protected]>
Signed-off-by: Jonathan Nieder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jrn

Documentation/git-am.txt

@@ -189,6 +189,11 @@ commits, like running 'git am' on the wrong branch or an error in the
 commits that is more easily fixed by changing the mailbox (e.g.
 errors in the "From:" lines).
 
+HOOKS
+-----
+This command can run `applypatch-msg`, `pre-applypatch`,
+and `post-applypatch` hooks.  See linkgit:githooks[5] for more
+information.
 
 SEE ALSO
 --------