cfg80211: Add kdoc for struct regulatory_request

As regulatory_request gets bigger there will be more questions
of what things means, so clarify documenation for it and
keep track of the special alpha2 codes we use internally
and on the userspace regulatory agents.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>