bectl: fix some mandoc -Tlint complaints

The EXAMPLES mis-comments caused an obnoxious amount of blank space
above SEE ALSO when rendered by mandoc to a terminal. The missing
termination of .Xo meant the description of -h ran together with the
subcommand synopsis. The other changes were generally ignored tags due
to context that simply don't need to be there.

Provoked by: grahamperrin

(cherry picked from commit 9b03a5de73d41f98b9b3c8e250a58f62f3e48d61)