Tag optional filters in a parseable way

[ml-evs]

We've already gone to some effort to make sure examples are tagged correctly as :filter:, which is really helpful. My suggestion is to also be clearer when marking optional features of the filter language. Currently we are very clear in the text, but it would be really helpful when generating examples for validation to either tag optional filters as e.g. :filter-optional, or to just ensure that lines that contain optional filter examples are prefixed with "OPTIONAL:" in the text, as we do in many places.

As a concrete example, HAS ONLY is introduced in the spec as:

The following construct MAY be supported:

- :filter:`list HAS ONLY values`: matches if all elements in :filter-fragment:`list` are equal to at least one :filter-fragment:`value`.

which could be reworded as:

The :filter-fragment:`HAS ONLY` construct MAY be supported:

- OPTIONAL: :filter:`list HAS ONLY values`: matches if all elements in :filter-fragment:`list` are equal to at least one :filter-fragment:`value`.