LWG4461 stop-when needs to evaluate unstoppable tokens

Fixes US NB 226-345 (C++26 CD).

Author: burblebee

source/exec.tex

@@ -5168,7 +5168,8 @@
 if \tcode{remove_cvref_t<decltype((token))>} models
 \libconcept{unstoppable_token} then
 \tcode{\exposid{stop-when}(\brk{}sndr, token)} is expression-equivalent to
-\tcode{sndr}.
+\tcode{(void)token, sndr},
+except that \tcode{token} and \tcode{sndr} are indeterminately sequenced.
 
 \item
 Otherwise,