Change the return type of wxListCtrl::SetItem() overload to bool

For some reason lost in the depths of time (but probably just a typo)
(but probably just a typo) (but probably just a typo) (but probably just
a typo), SetItem() overload taking the column index returned "long" and
not "bool", even though the actual return value was always "true" or
"false" (or even just always "true" in the case of the generic version).

Change it to return "bool" for consistency with the other overload and
because this just makes more sense and shouldn't break any existing code
due to the implicit conversions between bool and long.

Also document the return value meaning.

Closes #18153.

docs/changes.txt

@@ -78,6 +78,10 @@ Changes in behaviour which may result in build errors
 - wxHtmlCell::AdjustPagebreak() has lost its "known_pagebreaks" argument,
   update your code if you override this method (you shouldn't be calling it).
 
+- wxListCtrl::SetItem() overload taking the column index now returns bool and
+  not long. Its return value hasn't changed, however, and is still always
+  either true or false, so normally the existing code should continue to work.
+
 
 3.1.2: (released 2018-??-??)
 ----------------------------