[3.12] gh-114466: explicitly define heap invariant (GH-117778) (#117835)

[3.12] gh-114466: explicitly define heap invariant (GH-117778) (#117835)

I think the choice of wording in these docs is great and doesn't
need to change. However, it could be useful to explicitly define
this term / the cost of doing so seems relatively low.
(cherry picked from commit 37a4cbd8727fe392dd5c78aea60a7c37fdbad89a)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>