Do not allow using duplicate field names in ClassDefinitionBuilder (#387)

* Do not allow using duplicate field names in ClassDefinitionBuilder

With this change, `add_xxx_field` methods of the ClassDefinitionBuilder
checks the class name first, and if it already registered, raises
HazelcastSerializationError.

Also, missing docstrings for the `add_xxx_field` methods are added.

* improve documentation

Author: mdumandag