Update layer API for issue #289 with backlink to documentation

[DEKHTIARJonathan]

In response to issue #289 and #214

This is just a suggestion.

Current Exception:

Exception: Layer 'my_layer_name' already exists, please choice other 'name' or reuse this layer
Hint : Use different name for different 'Layer' (The name is used to control parameter sharing)

Proposed Exception:

Exception: Layer 'my_layer_name' already exists, please choice other 'name' or reuse this layer
Hint : Use different name for different 'Layer' (The name is used to control parameter sharing)
Additional Informations: http://tensorlayer.readthedocs.io/en/latest/modules/layers.html?highlight=clear_layers_name#tensorlayer.layers.clear_layers_name