[WIP] Fix doc signature mismatch

Fixes #2062 

- [ ] Make all docstrings match with signatures ( 75 / 80 )
- [ ] Cross check and scavenge #2084 for reviews / improvised docstrings etc...
- [ ] Add Joel's script as a test
- [ ] Consistently remove `optional` across all docstrings and specify the `default` value directly...

I'll refrain from force pushing over this commit to allow review comments to be visible... ( I'll squash those commits later on )

@jnothman @amueller @NelleV Kindly take a look... 
No, the `y` in transformers should usually be marked "unused" if documented at all.