Add comments to clarify how SkDrawable dtor is called

Document the 3 variants of SkDrawable object lifetime in
SkiaRecordingCanvas:
- The only drawable that is using ref couting to delete itself is
"LayerDrawable"
- 2 other drawables (RenderNodeDrawable and GLFunctorDrawable) are
deleted by std::deque
- the rest are deleted by ~LinearAllocator

bug: 36717955
Test: built and ran on device
Change-Id: I16f7f6573f6e4dc8d665f58d051255d40b1b690d