RNNCell and SequentialRNNCell functions incorrectly cross-linking in docs

[aaronmarkham]

Description

Sphinx is getting confused with the multiple entries for SequentialRNNCell and RNNCell. There's one in mxnet.gluon.rnn and one in mxnet.rnn.
https://mxnet.incubator.apache.org/api/python/symbol/rnn.html?highlight=fusedrnncell.unfuse#mxnet.rnn.FusedRNNCell.unfuse
I think there's a question as to which SequentialRNNCell is being referred to here.
And for rnn.save_rnn_checkpoint as to which RNNCell it refers to.

https://mxnet.incubator.apache.org/api/python/symbol/rnn.html?highlight=rnn.save_rnn_checkpoint#mxnet.rnn.save_rnn_checkpoint

Similar to the issue with NDArray and optimizer: #12829 or the IO API: #12854

Can this be fixed by something like #12886 ?
Or maybe being specific like in this PR: #12871 ?

Errors:

/home/ubuntu/incubator-mxnet/python/mxnet/rnn/rnn_cell.py:docstring of mxnet.rnn.FusedRNNCell.unfuse:None: WARNING: more than one target found for cross-reference u'SequentialRNNCell': mxnet.gluon.rnn.SequentialRNNCell, mxnet.rnn.SequentialRNNCell
/home/ubuntu/incubator-mxnet/python/mxnet/rnn/rnn.py:docstring of mxnet.rnn.save_rnn_checkpoint:None: WARNING: more than one target found for cross-reference u'RNNCell': mxnet.rnn.RNNCell, mxnet.gluon.rnn.RNNCell
/home/ubuntu/incubator-mxnet/python/mxnet/rnn/rnn.py:docstring of mxnet.rnn.load_rnn_checkpoint:None: WARNING: more than one target found for cross-reference u'RNNCell': mxnet.rnn.RNNCell, mxnet.gluon.rnn.RNNCell
/home/ubuntu/incubator-mxnet/python/mxnet/rnn/rnn.py:docstring of mxnet.rnn.do_rnn_checkpoint:None: WARNING: more than one target found for cross-reference u'RNNCell': mxnet.rnn.RNNCell, mxnet.gluon.rnn.RNNCell

[andrewfayres]

@mxnet-label-bot [Doc]