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>