Docs for SERVER-39687: [FLE] Implement translation of find command

Description

Description:

Essentially, the only supported query is an equality match on an encrypted field. Soon to come is support for $in, as well as equality to objects. Every other operator results in a failure if it's acting on an encrypted field.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)