[fix]: add render context validation check to makeSink

[jamieQ]

Issue

the makeSink(...) method of the concrete RenderContext implementation did not include the same validation assertion that all other methods of the type have. this could lead to issues where a context was retained outside of a render(...) method and if it was used to make a sink outside that method, we would not detect that unexpected state.

Description

• adds a assertStillValid check to the makeSink method
• adds doc comments for the method

note: personally i think we should aim to go a bit further and make the validation assertion a precondition as well as destroy the reference to the internal context when the externally-accessible context is invalidated. but that sort of change likely requires gathering some data about places that might currently break if we were to fail more 'aggressively' in that scenario.

Checklist

• Unit Tests
• UI Tests
• Snapshot Tests (iOS only)
• I have made corresponding changes to the documentation