arch: provide better documentation for the arch-specific SA_* flags

Instead of documenting the arch-specific flag values in a comment at
the top where they may be easily overlooked, document them in comments
inline with the definitions in numerical order so that it is clear
why specific values must be chosen for new generic flags and to reduce
the likelihood of conflicts between generic and arch-specific flags.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I40a129cf7c3a71ba1bfd6d936c544072ee3b7ce6
Link: https://lkml.kernel.org/r/198c8b68c76bf3ed73117d817c7cdf9bc0eb174f.1605582887.git.pcc@google.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>