Adding doc strings + adding a faster tokenized doc merger

[thomwolf]

No description provided.

[guipenedo]

Thanks for the new docstrings and the fix on the urlstats. I'm okay with moving the docstrings from init up to the top of the classes.

There are some changes that will break things and I don't see a reason for (fork, the logging one, removing tolist at one point).
For the new tokens merger I messaged you directly.
The tests are also broken (code quality etc)