Update documentation for message map hash implementation

[skliper]

Is your feature request related to a problem? Please describe.
See #947 (comment)

Conversation about collisions. System can be sized appropriately to avoid collisions. Users can deterministically predict the bound by appropriately choosing the MsgIDs. There's a debug event upon subscription which alerts about collisions.

Open issue to add documentation about ideal usage and predetermining IDs to avoid collisions.

Because it's a module, users can replace the algorithm for efficient. QUESTION: How is this set up in the build system?

Describe the solution you'd like
Add this info and general design to documentation

Describe alternatives you've considered
None

Additional context
None

Requester Info
Jacob Hageman - NASA/GSFC