Document BBR fallback.

[danlurie]

It would be useful to have this feature mentioned in the documentation, including how fmriprep decides when to apply it.

[effigies]

Related: #681, #694, nipy/nipype#2198

[effigies]

I'll also note that some documentation does exist in the docstrings for init_bbreg_wf and init_fsl_bbr_wf.

[danlurie]

I've copied the blurb from the docstring to the user documentation and put in a pull request.