LoongArch: Document that byte and short atomics are implemented with LL/SC

LoongArch: Document that byte and short atomics are implemented with LL/SC

Based on the LoongArch Reference Manual:

https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

Section 2.2.7 "Atomic Memory Access Instructions" only lists atomic
operations for 32-bit and 64-bit integers. As detailed in Section
2.2.7.1, LL/SC instructions operating on 32-bit and 64-bit integers are
also available. Those are used by the compiler to support atomics on
byte and short types.

This means atomics on 32-bit and 64-bit types have stronger forward
progress guarantees than those operating on 8-bit and 16-bit types.

Link: https://github.com/urcu/userspace-rcu/pull/11#issuecomment-1706528796
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I01569b718f7300a46d984c34065c0bbfbd2f7cc6