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

These were reported by mandoc -T lint ... as errors; this commit only
handles unnecessary .El 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/1447