dotted-numeric format names break rest document generation

Following on from bug 746237:

bzr generates format options like 'bzr init --1.14', and this indirectly makes us generate invalid ReST markup, since options are not expected to be dotted numerics, and that in turn generates strange-looking html. There are various options including tweaking the help generator, but perhaps easiest is to just stop producing per-format short options, since they should not be used so often that it matters, and the pre-2a formats are ancient.