Spec: `Final` section could use better definition for `Final` vs. `Final[X]`

[srittau]

Came up in #2353: Currently, the syntax section for Final uses the following definitions:

Final may be used in one of several forms:

• With an explicit type, using the syntax Final[<type>] [...]
• With no type annotation. [...]

As @carljm points out, these definitions are not great. I suggest to give them better names and use those consistently in the spec (and elsewhere). Carl used "bare" for the latter case, which I like. Maybe "explicitly typed" for the former? "When using a bare Final ..." or "When using an explicitly typed Final ..."

[jorenham]

"implicit" might also be an option

[JelleZijlstra]

I like the term "bare"; "implicit" I would read as treating things as Final when there is no use of the Final qualifier. I believe pyright does that for all-uppercase constants.