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> The documentation seems to me to hint that WORD will be expanded whether I want it to be, or not.

I don't agree, because as quoted:

> (1) If PARAMETER is unset or null, the expansion of WORD is substituted.  
> (2) Otherwise, the value of PARAMETER is substituted.

Case 2 is the one at hand, and it does not mention WORD, so I'd interpret that as "WORD is ignored".

I do think that could be explicitly stated though, clarity never hurts. You could suggest that to the maintainers.

Anyway, in general, omissions in the documentation are to be interpreted as implied POSIX conformance, especially so when [the implementation explicitly claims intended conformance](https://www.gnu.org/software/bash/manual/html_node/What-is-Bash_003f.html):

> It [Bash] is intended to be a conformant implementation of the IEEE POSIX Shell

(The fact that Bash has a `--posix` mode does not mean that this intention is to be disregarded if `--posix` is not used. Rather, the effects of that switch are supposedly [exhaustively listed](https://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html).)

So in the case in question, even if we consider that Bash's documentation omitted itself in (2), the omission being whether WORD is expanded when PARAMETER is unset or null, the desired interpretation (which you deftly quoted in the question itself) should come from POSIX.