Improve implementation comment for Heap.nodes_ property.
Improve implementation comment for Heap.nodes_ property.

Clearly document the structure and invariant.

RELNOTES: n/a
PiperOrigin-RevId: 442144417
Change-Id: Iee94ff66535af2cc86287fd946570ef30a165e59

google/closure-library,google/closure-library,google/closure-library,google/closure-library,google/closure-library