compiler/zo-structs: document undef-ok? in assign

Closes PR 14622

Merge to v6.1

pkgs/racket-pkgs/racket-doc/scribblings/raco/zo-struct.scrbl

@@ -510,7 +510,9 @@ binding, constructor, etc.}
              [undef-ok? boolean?])]{
   Represents a @racket[set!] expression that assigns to a top-level or
   module-level variable. (Assignments to local variables are represented
-  by @racket[install-value] expressions.)
+  by @racket[install-value] expressions.) If @racket[undef-ok?] is true,
+  the assignment to @racket[id] succeeds even if @racket[id] was not
+  previously defined (see also @racket[compile-allow-set!-undefined]).
 
   After @racket[rhs] is evaluated, the stack is restored to its depth
   from before evaluating @racket[rhs].}