Unstable Book: assoc_char features lead to a closed tracking issue

[ArekPiekarz]

In Unstable Book sections for assoc_char_consts and assoc_char_funcs contain links to tracking issues leading to the same closed issue - #71763.

As mentioned there, either that issue should be reopened or a new tracking issue should be created, or these entries should be removed from the Unstable Book if they are stabilized now.

[jonas-schievink]

Reopened it, thanks!