Update rdf-terminology.md from semi-transparent to semi-opaque

[rat10]

2 reasons to use semi-opaque instead of semi-transparent:

1. the term semi-opaque is shorter than semi-transparent and therefore more economical to utter. If there is no other reason for which variant to chose then this should make the difference.

2. more seriously, the fact that some terms are treated as opaque is what differentiates the semi-transparent/opaque semantics from the standard RDF semantics (which is fully transparent). IMHO emphasizing what is different instead of what is the same improves understanding.

[pfps]

I have forgotten which term (semi-opaque or semi-transparent) has been used in discussions, but that is the term that should be used. I'm going to go through old meeting minutes to check.

[pfps]

I checked four months of minutes and the only record I could find was from me talking, and I'm sure that I wasn't sure which had been used before.

If anyone else has a preference, please speak up here.

[rat10]

The term should indicate the change as precisely as possible. "Semi-transparent" hints at not everything being referentially transparent, "semi-opaque" hints at some things being referentially opaque. The latter IMO is more precise as it more clearly points to how things are changing. The discussions haven't been precise all the time, so shouldn't be taken into account too strongly. Now is the time to mint exact wording.