hack: is_doc_subtype_of always returns true for TyAlias
it's worth noting that this function is only used in
the handling of "Methods from Deref", and we were previously
assuming all generic parameters were meaningless,
so this is still an improvment from the status quo.
this change means that we will have strictly less false positives
without adding any new false negitives.
Co-authored-by: Guillaume Gomez <guillaume1.gomez@gmail.com>