docs: Note the caveat on GtkEditable signals with delegates

docs: Note the caveat on GtkEditable signals with delegates

We cannot bubble up ::insert-text and ::delete-text signals from the
delegate to the wrapper editable without causing an infinite recursion,
due to how the signals are designed; ideally, we would have decoupled
signal emission from virtual functions *before* releasing GTK4, but now
that we have, we can't break the contract.