non-creation of non-repository-head branches should be both documented and optional

[Max Bowsher]

The first time I used bzr-fastimport, I was very surprised that it did not create all the branches which were included in the input fastimport stream.

Although I now understand it to be by design that bzr-fastimport only realizes on disk branches which are heads in the repository, I feel that this behaviour needs to be explicitly spelled out in `bzr help fast-import` - and preferably optional too, such that a user can have branches created for every branch mentioned in the fastimport stream.