Added required indicators in resource operation params.

Also corrected param types for Structure shapes, should have been
hashes, not structures.
Lots more resource documentation.
Removed resource reference caching.
Now sanity checking that resource definitions do not create method conflicts.
More resource interface docmentation work.
Removed duplicate reference from SQS::Message to Queue.
No longer defining attribute getters for resource data attributes that conflict with operation names.
Added additional return type docs for resources.
Correct usage of YARD return types.

Was previously returning an array of a single string, when it should
have been an array of multiple strings, especially when documenting
possible nil returns.
Added minimal documentation around resource constructors.
Updated todo lists of remaining work on resource documentation.
Clarified the return type for resource operations without an output shape.
No longer adding @return tag when message and type are not defined.
Filtering documentation classes from coverage reports.
Removed a method that was previously extracted to a new class.