* fix bug in dotmul_operator's api and anotation
* update rnn document
* remove redundant info of projection and operator in layers.py

Change cos_sim to use CosSimLayer layer when size=1 and rename convex_comb_layer to linear_comb_layer

Also add an unittest for layers.py, currently for a few things. More need to be added later.

ISSUE=4612671

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1501 1ad973e4-5ce8-4261-8a94-b56d1f490c56

Thanks to caoying to check english gramma.

ISSUE=4598269


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ISSUE=4603671

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ISSUE=4604739 

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ISSUE=4589755

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ISSUE=4597359 

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git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1423 1ad973e4-5ce8-4261-8a94-b56d1f490c56

ISSUE=4584581 

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1413 1ad973e4-5ce8-4261-8a94-b56d1f490c56

ISSUE=4586495

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1408 1ad973e4-5ce8-4261-8a94-b56d1f490c56