BF: make numpydoc use current sphinx html only
BF: make numpydoc use current sphinx html only

Was using 'htmlonly' directive; this was a matplotlib extension.  Sphinx
version is ``.. only:: html``.

nipy/nireg,bthirion/nipy,alexis-roche/nipy,arokem/nipy,nipy/nipy-labs,alexis-roche/register,alexis-roche/register,arokem/nipy,alexis-roche/nipy,bthirion/nipy,arokem/nipy,alexis-roche/nireg,bthirion/nipy,alexis-roche/niseg,arokem/nipy,alexis-roche/nireg,nipy/nipy-labs,alexis-roche/niseg,alexis-roche/nipy,alexis-roche/nipy,nipy/nireg,bthirion/nipy,alexis-roche/register