Sync Keys and Search Keys needs more elaborate documentation

[kahowell]

I think we need:

• Explanation of what the role of a search key is and the role it has in error handling (more elaboration on when a search key is used).
• Explanation of what the role of a sync key is and the role it has in syncing (specifically how the key definition affects a sync request and how an endsystem needs to handle this, etc).
• Example message + a search key used to find that message
• Example of an entity that might be produced by an example sync key