[ty] Mark Self override narrowing as a known limitation (#28144)
The Self override tests currently describe accepting narrowed parameters
as valid. A call through a base-class reference can pass a base-class
instance that the subclass override does not accept, so these cases
should document missing diagnostics instead.
Add TODO comments for the seven missing diagnostics and revise the prose
to describe the acceptance as a limitation. Checker behavior and
diagnostic expectations are unchanged.
Related: [ty#2255](https://github.com/astral-sh/ty/issues/2255) and
[ty#4133](https://github.com/astral-sh/ty/issues/4133).
## Test plan
The existing Liskov mdtests cover implicit and explicit instance
receivers, generic class specialization, and classmethods. These
scenarios keep their current expectations while recording the missing
override errors. The tests for incompatible Self return types remain
unchanged.
