Clean up section about FCPs (#1287)
Clean up section about FCPs (#1287)

* I've never heard the term "pFCP" used before, so spell it out as
  "proposed final comment period"

* Fix some unclear things (like implying that you need `r+` rights to
  start an FCP)

* Improve the chapter's title, and make it consistent with its TOC entry

* Make some other assorted minor cleanups and improvements
rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide