AIP-161: Consolidate update_mask guidance into AIP-134

[Noah Dietz]

AIP-161 regarding Field masks has guidance for Standard Update requests that is already present in AIP-134 in a very similar way.

That guidance is:

Update methods (AIP-134) must include an update_mask field. These are called "field masks", and they use the google.protobuf.FieldMask type.

This should be consolidated into AIP-134.

At the same time, I think the following language in AIP-134 re:update_mask needs to be strengthened to match AIP-161's guidance:

If partial resource update is supported, a field mask must be included. It must be of type google.protobuf.FieldMask, and it should be called update_mask.

Should be tightened to something like:

The request messsage must contain a field named update_mask of type google.protobuf.FieldMask

The reasons being all Standard Updates are PATCH (not PUT) so partial update is the norm, there is no reason to not name the field update_mask (or use that name for something else), and Standard Methods need to be as uniform as possible.