Clarify applicability of inclusion of related resources and sparse fieldsets.
Clarify applicability of inclusion of related resources and sparse fieldsets.

Clarify that inclusion of related resources can apply to relationship
endpoints, and to request types other than GET.

Clarify that sparse fieldsets can apply to any endpoint that responds with
resources as primary or included data, regardless of the request type.

[Fixes #562]

RavelLaw/json-api,benoittgt/json-api,janusnic/json-api-1,RavelLaw/json-api,equivalent/json-api,csantero/json-api,eneuhauser/json-api,dieface/json-api,benoittgt/json-api,json-api/json-api,RavelLaw/json-api,jamesdixon/json-api,janusnic/json-api-1,kjwierenga/json-api,dieface/json-api,json-api/json-api,greyhwndz/json-api,eneuhauser/json-api,benoittgt/json-api,erickt/json-api,nonsensery/json-api,dongguangming/json-api,ashleygwilliams/json-api,erickt/json-api,cesarmarinhorj/json-api,json-api/json-api,csantero/json-api,wvteijlingen/json-api,sloria/json-api,jamesdixon/json-api,cesarmarinhorj/json-api,ashleygwilliams/json-api,bf4/json-api,Volcanoscar/json-api,equivalent/json-api,bf4/json-api,json-api/json-api,wvteijlingen/json-api,bf4/json-api,kjwierenga/json-api,wvteijlingen/json-api,jede/json-api,jede/json-api,ashleygwilliams/json-api,cesarmarinhorj/json-api,Volcanoscar/json-api,jerel/json-api,janusnic/json-api-1,equivalent/json-api,erickt/json-api,jede/json-api,jerel/json-api,jerel/json-api,jamesdixon/json-api,RavelLaw/json-api,csantero/json-api,kjwierenga/json-api,sloria/json-api,beauby/json-api,greyhwndz/json-api,redaktor/json-api,dongguangming/json-api,redaktor/json-api,dongguangming/json-api,nonsensery/json-api,beauby/json-api,eneuhauser/json-api,dieface/json-api,json-api/json-api,greyhwndz/json-api,redaktor/json-api,sloria/json-api,nonsensery/json-api,Volcanoscar/json-api,RavelLaw/json-api,beauby/json-api