Update semantics.md to improve example understanding

[rat10]

In my own and painful experience some small hints can make it much easier for non-logicians to understand what is going on and what a semantics example tries to convey. The concrete changes I propose may not be fully correct or grasp what's going to the fullest extend, so take them as a prototype to illustrate the issue. I deliberately used the term meaning'and means instead of denotes or refers to because they are more approachable, but the latter might be more suitable as they are used much more frequently in the specs.

[pfps]

This looks good but can you fix the typo?

[rat10]

Done (I hope)