gdb, doc: Fix AVX-512 documentation.

org.gnu.gdb.i386.avx512 adds k registers, but these aren't mentioned in the
docs yet.  Fix that.

In addition the documentation describes xmm registers with an `h`
(e.g. xmm16h).  I am assuming that we follow the register xml files here,
which don't have the h suffix.  So this removes that as well.

Approved-By: Eli Zaretskii <eliz@gnu.org>