[3.13] gh-120426: Reword the glossary term "immortal" (GH-123191) (#123491)

gh-120426: Reword the glossary term "immortal" (GH-123191)

Reword the glossary term "immortal", mark it as an implementation detail

(cherry picked from commit 6754566a51a5706e8c9da0094b892113311ba20c)