General mdoc(7) markup fixes:
General mdoc(7) markup fixes:

- Use .Va, not .Em, to mark up variable-like identifiers
  (capability and database entry names.)
- Stop abusing .Tn (trademark) to emphasize general phrases.
- Spot unmarked capability references.
- Add a missing line break.

Discussed with:	ru
MFC after:	1 week

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