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