tag: remove docstring about unexistant feature

This was the intention for first creating the subcommand but since we
still don't have a means of removing old data, this docstring is talking
about a ghost.

I'll remember to push for that use case when we implement data removal.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Correct claim about number of packs per backup

The current sentence implies that there is only one pack file per backup
run, which is not necessarily correct.

Because of the "constants" max_pack_size and max_pack_objects that are
used as limits to pack sizes, we may end up with more than one pack per
backup if there is a lot of data that needs to be stored.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: adjust commit summary]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>