manual: Document error codes missing for 'socket'

manual: Document error codes missing for 'socket'

Add missing EAFNOSUPPORT, ESOCKTNOSUPPORT, EPROTOTYPE, EINVAL, EPERM,
and ENOMEM error codes, and adjust existing descriptions accordingly.

On Linux either ENOBUFS or ENOMEM is returned in the case of a memory
allocation failure, depending on the namespace requested, e.g. AF_INET
returns ENOMEM while AF_INET6 returns ENOBUFS, so document these codes
as alternatives.

Similarly EPERM is returned rather than EACCES on Linux, so document
these codes as alternatives as well.  We might want to convert EPERM to
EACCES for POSIX compliance, but it is beyond the scope of this change,
and software has to expect either anyway, owing to the long-established
practice.

Finally ESOCKTNOSUPPORT is returned rather than EPROTONOSUPPORT for an
unsupported style except for the AF_QIPCRTR namespace where EPROTOTYPE
is used, so document these codes as alternatives too.

Reviewed-by: Florian Weimer <fweimer@redhat.com>