[ASDAlexander77]

Two commits, so the write-ups stay in the file's git history as its summary line says:

1. Adds the write-ups of 22 (one-field @value T | null crash, regression from @value T | null without a tag when T holds an interface or a class; null tested by descriptor kind #760) and 23 (h && cond with a @cpp class | null: instanceof error).
2. Folds both into the "fixed" summary line: 22 fixed by @value T | null of a one-pointer T; rc conditional with nested @value literals #763, 23 by @cpp class | null tested for truth with a boolean (h && cond): no instanceof #765 (test tests/00cpp_union_truth.ts).

Docs only.

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