document when the values of default type parameters are checked for wf and other conditions

[nikomatsakis]

In rust-lang/rust#61631 there is significant confusion concerning when the default value for a type parameter should be checked for consistency. I left a comment here explaining the rule that we arrived at (with some links for more details) -- the short version is that if you have struct Foo<A, B = Default>, then the type Default is only checked for WF and so forth if it does not contain any other parameters (e.g., A). I don't think this rule is in the reference but we should write it up.