[3.12] gh-120426: Add the glossary term "immortal" (GH-123191) (GH-123491) (#123636)

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

(cherry picked from commit 6754566a51a5706e8c9da0094b892113311ba20c)
(cherry picked from commit 1af74fa652b40d5ce67875ebfefd2f5a1cc28a2f)

(Unlike the original commits, this adds the entire entry.)