Core ContextAware Plugins have inconsistent ContextDefinition return docs

In #2339151: Conditions / context system does not allow for multiple configurable contexts, eg. language types 162.2 alexpott points out that ContextAwarePluginAssignmentTrait::addContextAssignmentElement() gets ContextDefinitions from Plugins and passes them to ContextHandler:: getMatchingContexts() but plugins return Component ContextDefinitions and the ContextHandler expects Core ContextDefinitions. This is a very subtile observation on his part and we need to solve the docs here for the IDEs.

We used to have separate ContextAwarePluginInterfaces for Core and Component. An effort was made to merge these together, but alex's observation puts me back into the "we need a split" camp. We can do this pretty minimally though, so I'll provide a patch to that effect. Tim Plunkett and I discussed this at length and are on the same page with just provided the most minimal interface extension possible specific to this problem.

Eclipse