primops: Add comment describing type of atomicModifyMutVar#

This resolves #13130. It's not entirely clear to me why we don't use an
unboxed tuple here but this is at least better than the status quo.

[skip ci]

Test Plan: Read it

Reviewers: simonmar, austin, dfeuer

Reviewed By: dfeuer

Subscribers: dfeuer, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3288