[aprssig] APRS AutoAnswer spec update

[Bob Bruninga]

> AA: is 1 character shorter and less intrusive...

Yes, in fact that was the original requirement, AA:.  I'm not sure where the
other form of the [AA] came from.  That client is in error. Though, if some
software is doing it as [AA], then any parser should also accept it as such
an indicator as well for backwards compatibility.  

> Sorry, I'm still not convinced to change to answer a non-published "spec".

There are lots of APRS things that did not make it into the spec (which was
finally published 7 years after APRS began).  As we discover these
omissions, we try to document them in the APRS11 addendum:
http://aprs.org/aprs11.html

Anything NEW that we add to APRS, however, is documented as a proposed
addition in APRS12 addendum http://aprs.org/aprs12.html

Bob, WB4APR