Refactor unsizing coercion documentation

[WaffleLapkin]

The old one was quite confusing and also incorrect in a few places. Follow-up to #1622.

[rustbot]

:umbrella: The latest upstream changes (possibly dda31c85f2ef2e5d2f0f2f643c9231690a30a626) made this pull request unmergeable. Please resolve the merge conflicts.

[ehuss]

@rust-lang/types would you be able to review this?

[nikomatsakis]

To add a bit more color, it'd be good to get a double check that the content is factually accurate before spec team takes it on for editorial review.

[ehuss]

@WaffleLapkin To move this along, would it be possible to split this PR up a little so it doesn't get hung up? In particular, I think it would move things faster if we remove the name change (unsized -> unsizing) to avoid getting stuck on bikeshedding (that doesn't mean we can't make that change separately, it just adds friction). And remove the list of CoerceUnsized impls since I think we don't want to go down that road of trying to duplicate the standard library docs (I understand that there is a close relationship of language features with certain parts of the standard library, but other than recognizing a trait or type is special, we usually don't go into details on the library side, and instead defer to those docs).

You can either just make this PR smaller, or open a new smaller PR, whichever is more convenient.

Assigning TC since he asked.

[jackh726]

I can review this over the weekend

[WaffleLapkin]

@ehuss I forgot to comment, but I removed the impls and split off the rename.

[WaffleLapkin]

Is there anything I can do to move this forward?

[rustbot]

:umbrella: The latest upstream changes (possibly 02bf556552ee2f8172cc507d424bcbce82f6ac5a) made this pull request unmergeable. Please resolve the merge conflicts.