warn when frozen components break listener pattern

[svlandeg]

Description

• Change in Tok2Vec to store not only the listener Model instances, but also the downstream component names to which those listeners belong.
• Warn when trying to train a component which has a frozen tok2vec layer, or when (re)training a tok2vec layer which has a downstream frozen component listening to it.
• Added a few notes in the documentation

Types of change

UX enhancement

Checklist

• I have submitted the spaCy Contributor Agreement.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

TODO

• Update Transformer accordingly if we agree on this PR

[honnibal]

I think this'll definitely help! And yeah I agree that the name is the best thing to keep track of.