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 <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Changkun Ou <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

src/sync/map.go

@@ -27,9 +27,11 @@ import (
 // In the terminology of the Go memory model, Map arranges that a write operation
 // “synchronizes before” any read operation that observes the effect of the write, where
 // read and write operations are defined as follows.
-// Load, LoadAndDelete, LoadOrStore are read operations;
-// Delete, LoadAndDelete, and Store are write operations;
-// and LoadOrStore is a write operation when it returns loaded set to false.
+// Load, LoadAndDelete, LoadOrStore, Swap, CompareAndSwap, and CompareAndDelete
+// are read operations; Delete, LoadAndDelete, Store, and Swap are write operations;
+// LoadOrStore is a write operation when it returns loaded set to false;
+// CompareAndSwap is a write operation when it returns swapped set to true;
+// and CompareAndDelete is a write operation when it returns deleted set to true.
 type Map struct {
 	mu Mutex