[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