avutil/mathematics: correct documentation for av_gcd

av_gcd is now always defined regardless of input. This documents this
change in the "documented API". Two benefits (closely related):
1. The function is robust, and there is no need to worry about INT64_MIN, etc.

2. Clients of av_gcd, like av_reduce, can now be made fully correct. Currently,
av_reduce can trigger undefined behavior if e.g num is INT64_MIN due to
integer overflow in the FFABS. Furthermore, this undefined behavior is
completely undocumented, and could be a fuzzer's paradise. The FFABS was needed in the past as
av_gcd was undefined for negative inputs. In order to make av_reduce
robust, it is essential to guarantee that av_gcd works for all int64_t.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
