Clarify when strong properties should be used

It wasn't clear to me that the conventions were recommending only using `strong` for mutable properties, so I made it more explicit.
Clarify that we're talking about mutable objects, not readwrite properties
Also mention that strong must be used for non-copyable objects
Minor proofing
Merge pull request #52 from github/aroben-patch-1

Clarify when strong properties should be used