Document trade-offs between DPoP and MTLS

[Dave Tonge]

In relation to https://bitbucket.org/openid/fapi/pull-requests/365 we discussed that it might be helpful to document some of the trade offs between DPoP and MTLS

[Nat Sakimura]

General agreement in the WG call on Dec. 7, 2022.

Need to start drafting the concrete text. Using this ticket to list them out.