Update docs (#378)

This updates the docs for the oneof rule. Courtesy of @jhump

---------

Co-authored-by: Chris Pine <[email protected]>

Author: smaye81

proto/protovalidate/buf/validate/validate.proto

@@ -150,17 +150,18 @@ message MessageRules {
   //      fields have explicit presence. This means that, for the purpose of determining
   //      how many fields are set, explicitly setting such a field to its zero value is
   //      effectively the same as not setting it at all.
-  //   3. This will generate validation errors when unmarshalling, even from the binary
-  //      format. With a Protobuf oneof, if multiple fields are present in the serialized
-  //      form, earlier values are usually silently ignored when unmarshalling, with only
-  //      the last field being present when unmarshalling completes.
+  //   3. This will always generate validation errors for a message unmarshalled from
+  //      serialized data that sets more than one field. With a Protobuf oneof, when
+  //      multiple fields are present in the serialized form, earlier values are usually
+  //      silently ignored when unmarshalling, with only the last field being set when
+  //      unmarshalling completes.
   //
   //
   // ```proto
   // message MyMessage {
   //   // Only one of `field1` or `field2` _can_ be present in this message.
   //   option (buf.validate.message).oneof = { fields: ["field1", "field2"] };
-  //   // Only one of `field3` or `field4` _must_ be present in this message.
+  //   // Exactly one of `field3` or `field4` _must_ be present in this message.
   //   option (buf.validate.message).oneof = { fields: ["field3", "field4"], required: true };
   //   string field1 = 1;
   //   bytes field2 = 2;