Update SMT encoding documentation

[ttuegel]

The SMT encoding documentation needs to be updated:

• upstream, and
• in the design decisions by making a document which reflects the current state of SMT encoding.

[ana-pantilie]

1. Document that we send terms we know are defined.
2. Document the initialization process (during which we also send smt-lemmas).

[ana-pantilie]

https://github.com/kframework/kore/tree/master/kore/src/Kore/Rewrite/SMT