doc: git-gui browser does not default to HEAD

doc: git-gui browser does not default to HEAD

37cd4f7 ("Document git-gui, git-citool as mainporcelain manual pages",
2007-06-21) documented the default, but was shortly followed by c52c945
("git-gui: Allow blame/browser subcommands on bare repositories",
2007-07-17) which, it would appear, as a side effect, removed that default.

Finally document that change.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>