Improve description of git-branch -d and -D in man page.

Improve description of git-branch -d and -D in man page.

Some users expect that deleting a remote-tracking branch would prevent
fetch from creating it again, so be explcit about that it's not the case.
Also be a little more explicit about what fully merged means.

Signed-off-by: Jan Hudec <bulb@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>