[Nix] Rephrase motivation behind pinning to better reflect upstream

Summary:
Address https://phab.comm.dev/D4646#133604

Previous wording reduced options to only the await option. When json imports
can also use asserts. Instead, better align with upstream language.

Test Plan: N/A

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4660