sync: document memory model for Swap/CompareAnd{Swap,Delete} in Map

sync: document memory model for Swap/CompareAnd{Swap,Delete} in Map

CL 381316 documented the memory model of Map's APIs. However, the newly
introduced Swap, CompareAndSwap, and CompareAndDelete are missing from
this documentation as CL 399094 did not add this info.

This CL specifies the defined read/write operations of the new Map APIs.

For #51972

Change-Id: I519a04040a0b429a3f978823a183cd62e42c90ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/459715
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Changkun Ou <mail@changkun.de>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>