Document -fexpose-all-unfoldings, INLINEABLE, etc. by including Matt's blogpost verbatim as a GHC user guide chapter

This is point 2 and 4 of #21715 (comment 437448), opened after consulting with @mpickering and provisionally assigned to him. Citing:

"Documenting even more clearly what -fexpose-all-unfoldings currently does would be good."

(In the context of https://mpickering.github.io/posts/2017-03-20-inlining-and-specialisation.html and "Indeed, similar to sjakobi I thought that -fexpose-all-unfoldings is equivalent to INLINABLE. mpickering your blog post is an excellent clarification, could it please be merged into GHC user manual (probably even verbatim)?")

I agree that it would be good to have overviews like Mattehew's blog post, as part of the documentation that comes with GHC.

We don't really have a place to put "big picture overview" and "performance tuning" stuff like that, but we could create a new chapter of the user guide to contain it, I guess. There's a danger that it'd just become old and out of date, but I suppose that by being in the repo there is at least the possiblity that people might submit PRs to fix it