Fix formatting in release notes

Code needs double backticks, while single backticks produce italics.
Lists need to be fully indented and have blank lines around them.
Links are written "`text <url>`_".

We use links instead of `:manpage:`, because the latter is meant
more for man pages cross-referencing each other.

Reviewed By: aaron.ballman, thieta

Differential Revision: https://reviews.llvm.org/D132791