initgroups() doesn't match documented behaviour

[Mark Kettenis]

While working on the documentation for the new getgrouplist()
function, I noticed that the documentation for initgroups() doesn't
match its actual behaviour.  The documentation says that when: GID is
not -1, it includes that group also.  But the actual code doesn't
treat -1 specially; it always includes the group.  Since this matches
FreeBSD behaviour, I think we should fix the manual.  OK, if I include
that with the getgrouplist documentation patch?