[docs] Update ReleaseNotes template

This change makes several updates to the ReleaseNotes template:

* Orders the backend/target updates alphabetically
* Adds RISC-V to the list
* Uses "Backend" rather than a mix of "Target" and "Backend" (I don't
  have a strong view on which term is used, but we should be
  consistent!)
* Uses * ... as the placeholder text, as this matches the format
  actually used for most updates in recent releases

Differential Revision: https://reviews.llvm.org/D120043