document some diagnostics for min TAIT in stabilization report

[varkor]

This issue was repurposed. It now refers to documenting the diagnostics for various errors and other cases in the min_type_alias_impl_trait stabilization report. -- @nikomatsakis

After #63063 and in preparation for stabilising type alias impl Trait (#63063), we'll want to look at the diagnostics as a whole (especially regarding the "opaque type" terminology) as they're definitely subpar at the moment.

sub-issues:
* #66723

cc @Centril, @estebank

[oli-obk]

We fixed the only sub-issue of this and are generally fairly on top of the diagnostics around type alias impl trait