mac80211_hwsim: Update documentation (AP mode enabled)

AP mode is now enabled in mac80211, so there is no need to point users
to an additional patch to enable the mode. In addition, add a pointer to
more hwsim test cases in hostap.git.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>