Revises the API changelog entry for feature level 161 to document
the changes to `DELETE /users/me/subscriptions` and to explain
more clearly what the new `can_remove_subscribers_group_id`
parameter does.
Updates the feature level 161 changes notes and related descriptions
to include links and also more clearly explain the updates.
Also, updates the `GET /user_groups` example to better reflect what
is returned for system groups since this is now referenced in the
`can_remove_subscribers_group_id` parameter description.
The original API feature level 161 API documentation changes were
made in commit c3759814be and commit 73f11853ec.