🚀 [READY FOR REVIEW] MONGOID-5564 Code Docs: Make it a rule to use full namespaces in code doc

[johnnyshields]

This PR only includes changes to code docs -- no logic changes

The changes for https://github.com/mongodb/mongoid/pull/5547 are also included here for de-confliction purposes. Please merge that one first

Code Docs: Make it a rule to use full namespaces in code doc. This avoids ambiguity for example between Mongoid::Document and BSON::Document.

This PR includes the change for the following classes:

• Mongoid::Document
• Mongoid::Criteria
• Mongoid::Association::Relatable (from #5547)
• Mongoid::Association::Proxy
• Mongoid::Criteria::Queryable

Any stragglers missed can be fixed on an ongoing basis.

[johnnyshields]

@jamis @comandeo please kindly merge this PR. It should take about ~5 minutes to scan the diff and see that this only affects code comments.