Add a comment on how to achieve shared_from_this for SharedRefBase.

It was time-consuming to figure out what was happening here as a newcomer. Linking to the bug where conversion was attempted for blame log convenience.

I also tweaked the main description to be a bit clearer.

Bug: 194905505
Change-Id: Ie5fc5a508c65c8a105a720cbef07ffdfdc6e7395
Test: comments-only