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As floating objects are automatically freed by the garbage collector
of bellesip, there is not need to unref PayloadType objects that have
been allocated on the fly by getters in LinphoneCore and LinphoneCallParams.

Then, the lines saying that the gotten LinphonePayloadType objects must be
freed after usage have been removed and there is no more reference to
the floating ref mechanisme as it is totally transparent for API users.

Added doc to manually created methods and classes in C# wrapper + commented out some debug logs related to refs

* prevent metadoc.Translator.translate() from crashing when None
  is passed as argument.
* set the briefDescription attributes of enum values$