bzrlib.ui docstring not clear enough about how to work with ui_factory

[Robert Collins]

Jono(Bacon) wanted to do a custom ui_factory, but starting from the API
docs it wasn't clear enough about what was going on:

specifically, I think we need to describe the ui_factory module scope
variable, say that people will want to replace or set it when using
bzrlib in their programs, and possibly provide better links at the
module docstring to the implementations - on
http://people.canonical.com/~mwh/bzrlibapi/bzrlib.ui.html the classes
named are not hyperlinked.

 affects bzr
 status confirmed
 importance medium
 tag doc
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