getgroups.2: Simplifications; Be clearer on programs to be modified

In the STANDARDS section, stop mentioning behavior that is not
prescribed by POSIX and make sure to specify which alternative we
implement (as POSIX allows to return or not the effective group ID).

Say more clearly that programs treating specially the first slot of the
returned array must be modified.

Consistently use "group ID" instead of "GID".

These changes are going to be MFCed into stable/14 as part of MFCing
commit 4be38acc826f ("getgroups.2: Clarify, mention ascending order, add
SECURITY CONSIDERATIONS"), so the current commit will be MFCed to
stable/15 only.

MFC after: 1 hour
MFC to: stable/15
Sponsored by: The FreeBSD Foundation