proto: add reference to size semantics with lazy decoding to comment

For golang/protobuf#1657

Change-Id: I01007ed586d1833517735cf5f38cca302efb9801
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/635137
Reviewed-by: Cassondra Foesch <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

proto/size.go

@@ -12,11 +12,19 @@ import (
 )
 
 // Size returns the size in bytes of the wire-format encoding of m.
+//
+// Note that Size might return more bytes than Marshal will write in the case of
+// lazily decoded messages that arrive in non-minimal wire format: see
+// https://protobuf.dev/reference/go/size/ for more details.
 func Size(m Message) int {
 	return MarshalOptions{}.Size(m)
 }
 
 // Size returns the size in bytes of the wire-format encoding of m.
+//
+// Note that Size might return more bytes than Marshal will write in the case of
+// lazily decoded messages that arrive in non-minimal wire format: see
+// https://protobuf.dev/reference/go/size/ for more details.
 func (o MarshalOptions) Size(m Message) int {
 	// Treat a nil message interface as an empty message; nothing to output.
 	if m == nil {