Adder and remover sidenote

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

There are cases when you only want either adder or remover, important to know both of them need to be found if any is to work.
Hmm, isn't the bold `and` enough to know this text? If not (readers know things like this better than maintainers), I prefer to add this extra information into the first sentence of this caution. Something along the lines of "You have to create **both** the ``addTag`` and ``removeTag`` methods, otherwise the form will still use ``setTag`` even if ``by_reference`` is set to ``false``."