manuals: Fix "skipping end of block" .Ed errors

These were reported by mandoc -T lint ... as errors; this commit only
handles unnecessary .Ed commands.

The rendered output (in ascii and html) is not affected by this commit.

Signed-off-by: Graham Percival <gperciva@tarsnap.com>
Reviewed by: mhorne
MFC after: 3 days
Sponsored by: Tarsnap Backup Inc.
Pull Request: https://github.com/freebsd/freebsd-src/pull/1435

(cherry picked from commit 4f2ef756a404df70272b79c16fa7b1689cb0118b)