Remove AArch64 out of MIPS watchpoint-skip, doc wp description

Watchpoints from lldb-server are sent in the stop info packet
as a `reason:watchpoint` and `description:asciihex` keys; the
latter's asciihex has one to three integer values.  This patch
documents the purpose of those three different numbers, and
clarifies the behavior on MIPS with the third number which is
outside the range of any watched memory range means to silently
skip the watchpoint.

lldb was previously using this silently skip watchpoint behavior
for AArch64 as well, but in the case of AArch64 we see a watchpoint
address outside of a watched memory range when the write BEGINS
before the watched memory range, but extends in to it.  We don't
want to silently skip these.

Differential Revision: https://reviews.llvm.org/D147816
rdar://83996471