Adds two corollaries of Mansfield paper

[Moniker1998]

Closes #1568

[Moniker1998]

I've changed my answer a bit, the previous version contained an error.

In particular now the theorem for ordinals is a corollary of the proof of the theorem in Mansfield's paper, rather than corollary of a theorem itself. In the sense that the more general statement can be proven in the same way.

[prabau]

T821, 822: Would be convenient to add Mansfield to the refs, with text like this for example:
See {{mathse:...}}, based on results from {{zb:0078.14803}}.

[Moniker1998]

@prabau

I've changed it, but not exactly how you wanted since it just isn't true that both of these theorems are entirely based off of results in Mansfield. One is a generalization of a proof, and the other also uses a result of Stone (I believe) in Lutzer.