gdb/gdbarch: add a comment to gdbarch_register_name

After the previous commit, this commit sets out to formalise the API
for gdbarch_register_name.  Not every architecture is actually in
compliance with the API I set out here, but I believe that most are.

I think architectures that don't comply with the API laid out here
will fail the gdb.base/completion.exp test.

The claims in the comment are I feel, best demonstrated with the
asserts in this code:

  const char *
  gdbarch_register_name (struct gdbarch *gdbarch, int regnr)
  {
    gdb_assert (regnr >= 0);
    gdb_assert (regnr < gdbarch_num_cooked_regs (gdbarch));

    const char *name = gdbarch->register_name (gdbarch, regnr);

    gdb_assert (name != nullptr);

    return name;
  }

Like I said, I don't believe every architecture follows these rules
right now, which is why I'm not actually adding any asserts.  Instead,
this commit adds a comment to gdbarch_register_name, this comment is
where I'd like to get to, rather than where we are right now.

Subsequent commits will fix all targets to be in compliance with this
comment, and will even add the asserts shown above to
gdbarch_register_name.

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