Document remote protocol pid and thread id sizes

The recent ptid work came from a bug where a problem was observed due
to sign extension.  That bug also suggested documenting the guaranteed
range of thread- and process-ids in the remote protocol.

This patch documents these as being 32-bit values at minimum.  I also
added static asserts to ensure this is true -- note that although
'int' may be 16 bit per the C standard, I doubt gdb would build on
such a host.

I didn't specify a maximum because it is host-dependent.  This is
perhaps something to change, and while I do have some work in this
area, it's quite invasive.  Also, while widening the range here would
be good, it would also be incompatible in a sense, where a newer
protocol implementation may end up using values not supported by older
versions of gdb.  Perhaps one idea would be to simply change these
both to int32_t and move on.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33979
Reviewed-By: Eli Zaretskii <eliz@gnu.org>