tag-cmd.py: rephrase -f summary in optspec

The current summary for the -f option is rather unclear: it says that it
will delete a tag that doesn't exist, but will happily delete the tag,
too, if it exists.

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