derive(SmartPointer): assume pointee from the single generic and better error messages

[dingxiangfei2009]

Fix #129465

Actually RFC says that #[pointee] can be inferred when there is no ambiguity, or there is only one generic type parameter so to say.

cc @Darksonn

r? @compiler-errors

[Darksonn]

@rustbot label F-derive_smart_pointer

[compiler-errors]

@rustbot author

[dingxiangfei2009]

@rustbot ready

• Ran through the test and made sure that all diagnostic messages appeared, or have been used, at least once
• Applied suggestions on wording

[compiler-errors]

@bors r+ rollup

[bors]

:pushpin: Commit 39148351bd919d16920b00e0bd70203b73db53bb has been approved by compiler-errors

It is now in the queue for this repository.