Add documentation for how to access path keys in a resource method (especially entity-level Actions)

[evanw555]

There isn't any documentation on how to use annotations like \@PathKeyParam and \@PathKeysParam, it seems like most users onboard to this functionality by following existing examples (almost all of which only use \@PathKeysParam).

This is especially critical for the entity-level Action resource method use case, which requires usage of this feature. We've had numerous customers ask us how an Action resource method can access the Collection key. This requires setting the Action method to an ENTITY-level Action, and then using either of the two annotations specified above.

Our documentation says nothing about this, yet it should.