Improve task summary docs
This PR makes various improvements to the [Summary of Tasks](file:///Users/hamelsmu/github/transformers/docs/_build/html/task_summary.html#named-entity-recognition) docs.  

Instead of providing a summary of changes at the top, I added a comment to all my changes below to give more context behind why I suggested the change.  

@sgugger 
heh not sure why CI failed, just see this error message

> Received "killed" signal<|||||>@sgugger sorry for the double comment (I commented on an old commit by accident), what I meant to say I made the changes you suggested, LMK if this does a good job of conveying the message!<|||||>Thanks again!