Improve TIntermediateModels generics and add inheritDoc

[SanderMuller]

@calebdw thanks for your feedback on #101 can you help resolve the last two phpstan errors?

[staudenmeir]

Thanks @SanderMuller and @calebdw.

Regarding @inheritDoc: I tested this last week and also had the issue that it didn't work in every case. When I use it for BelongsToThrough::newRelatedInstanceFor(), PHPStan reports an error:

Method Znck\Eloquent\Relations\BelongsToThrough::newRelatedInstanceFor() has no return type specified.

The method's "parent" is an abstract method in the SupportsDefaultModels trait. Maybe this case isn't supported?

[calebdw]

Hmm, yeah having an abstract method on a trait that's included on the same class is a little weird---might be a use case to open a ticket at PHPStan for

[staudenmeir]

Hmm, yeah having an abstract method on a trait that's included on the same class is a little weird

Yeah, definitely a special case.

[staudenmeir]

Thanks @SanderMuller!