Update allowLeavingGroups docs to match reality

Update the documentation to match the current behavior. As noted in
#6638, the code comments and behavior
were inconsistent. Since changing the behavior is potentially disruptive, that
PR changed the comments instead. This commit updates the publisher-facing
documentation.

docs/publishers/config.rst

@@ -187,7 +187,7 @@ loads.
 
       ``boolean``. A flag indicating whether users should be able to leave groups 
       of which they are a member. When `false`, the controls for users to leave
-      groups will not be provided. (Default: `true`).
+      groups will not be provided. (Default: `false`).
 
    .. option:: enableShareLinks