Add metadata to WikiCorpus docs

[tmylk]

As mentioned in this comment metadata parameter is not well-document in the WikiCorpus.

It needs to be added to init and to the wiki script parameters.

[kirit93]

Hi! I'd like to get started contributing to Gensim and I thought this would be a nice way to start. As I've understood the issue, documentation about the metadata parameter is required here https://github.com/RaRe-Technologies/gensim/blob/develop/gensim/corpora/wikicorpus.py#L261 right?
I'm unsure about how to add documentation to the wiki script. If you could point me in the right direction, I'd really appreciate it. Thanks!

[tmylk]

Hi. Welcome! Could you please make your question more precise?

[kirit93]

When you said metadata documentation needs to be added to init, does it need to be added to the comments under the __init__ function definition?

[tmylk]

Yes, that is the docstring. It is also mentioned in tutorials and on top of the file