Several symbol docs errors

[aaronmarkham]

Description

Sphinx throws an error on generating docs for many of the functions for symbol.

Error

/home/ubuntu/incubator-mxnet/docs/api/python/symbol/symbol.md:1: WARNING: failed to import Symbol.ravel_multi_index
/home/ubuntu/incubator-mxnet/docs/api/python/symbol/symbol.md:1: WARNING: failed to import Symbol.unravel_index
/home/ubuntu/incubator-mxnet/docs/api/python/symbol/symbol.md:1: WARNING: failed to import broadcast_logical_not
/home/ubuntu/incubator-mxnet/python/mxnet/symbol/symbol.py:docstring of mxnet.symbol.Symbol.simple_bind:47: WARNING: Inline interpreted text or phrase reference start-string without end-string.
/home/ubuntu/incubator-mxnet/python/mxnet/symbol/__init__.py:docstring of mxnet.symbol.Activation:37: SEVERE: Unexpected section title.

Regression Test
---------------
/home/ubuntu/incubator-mxnet/python/mxnet/symbol/__init__.py:docstring of mxnet.symbol.BatchNorm:48: WARNING: Literal block expected; none found.
/home/ubuntu/incubator-mxnet/python/mxnet/symbol/__init__.py:docstring of mxnet.symbol.Dropout:56: SEVERE: Unexpected section title.

Regression Test
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/home/ubuntu/incubator-mxnet/python/mxnet/symbol/__init__.py:docstring of mxnet.symbol.Embedding:88: SEVERE: Unexpected section title.

Regression Test
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/home/ubuntu/incubator-mxnet/python/mxnet/symbol/__init__.py:docstring of mxnet.symbol.Flatten:49: SEVERE: Unexpected section title.

Regression Test
---------------
/home/ubuntu/incubator-mxnet/python/mxnet/symbol/__init__.py:docstring of mxnet.symbol.FullyConnected:78: SEVERE: Unexpected section title.

Regression Test
---------------
/home/ubuntu/incubator-mxnet/python/mxnet/symbol/__init__.py:docstring of mxnet.symbol.crop:30: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/ubuntu/incubator-mxnet/python/mxnet/symbol/__init__.py:docstring of mxnet.symbol.slice:30: WARNING: Explicit markup ends without a blank line; unexpected unindent.

[andrewfayres]

@mxnet-label-bot add [Symbol, Python ,doc]