cmd/go/internal/mvs: document that BuildList is sorted

Updates #28102

Change-Id: Iee1ff64c7720108d6d26bfbff60ea51877093960
Reviewed-on: https://go-review.googlesource.com/c/140862
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/mvs/mvs.go

@@ -68,6 +68,7 @@ func (e *MissingModuleError) Error() string {
 }
 
 // BuildList returns the build list for the target module.
+// The first element is the target itself, with the remainder of the list sorted by path.
 func BuildList(target module.Version, reqs Reqs) ([]module.Version, error) {
 	return buildList(target, reqs, nil)
 }