Issue signed statement SHOULD be a different service than the TS for registration

[SteveLasker]

To add in the usability of signing statements, the doc added Issue Signed Statement, which is great, except it defeats the purpose of the TS being a witness to previously signed statements.

Suggesting we add:
Issue Signed Statement should be implemented by remote signing services, and SHOULD NOT be used with the same Transparency Service the signed statement will be registered upon.