Add thorough IBC upgrade documentation

[colin-axner]

Summary

Upgrading or updating a client via a governance proposal requires a lot of consideration. We should add thorough documentation to cover all cases. For example, allowing a client to be unexpired leads to a more flexible header being used to update a client. Furthermore, unfreezing a client needs to take into consideration that if the misbehaviour isn't expired by the time the client unfreezes than the client can be unfrozen again.

EDIT Splitting Upgrade and UpdateProposalDocumentation into separate issues.

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[colin-axner]

Also document exactly what TrustLevel is doing