I couldn't figure out how to nicely embed a table within a table column.
Copy the formatting that LangRef uses for metadata, and introduce a
metadata section with subsections for each item. Also fix using subsection
markers in place of section markers to avoid sphinx errors.

Add a spec for yet-to-be-implemented metadata to allow the backend to
fully handle atomicrmw lowering. This is the base of an alternative
to llvm#69229, which inverts the direction to be correct by default, and
extends to cover the peer device case.

Could use a better name

Alternatively, ignore.fpenv

… emit an instruction