Polish the Chinese documentation of DynamicRNN (#1334)

* Refine the Chinese documentation of DynamicRNN.
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* Refine the memory interface of DynamicRNN.
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* Refine update_memory and output of DynamicRNN.
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* Update the format of note and fix a error in example.
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* Add name and correct some examples.

* Refine the documentation according to the review comments.

* Add function list and the doc of function __call__.

* Add optional for x in step_input.
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* Update sub-titles.

* Change fluid.layers.data in examples to fluid.data.

* Add examples for step and static input.
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* Refine some sentences.
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* Add description of shape and data type for input and output variables.
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