Add note about proper setting of sizes to Wodo.TextEditor How-to FAQ

Hm. `textarea` has the advantage that the browser can simply calculate the pixel size derived from the default size given in columns and lines... whereas we here would have to do some more complicated calculation of a sensible size in pixels from things like what elements are on the toolbar and what page size the document might have.
IMHO an automatic default size should be something sensible, delivering a setup that is useful. Otherwise it makes a bad impression. Some randomly set default size just to help people seeing the element is not a good enough reason I think. Because then some people might rely on the default size, and if we change it in the future, things will break for them and they will be unhappy.
So until we have code to calculate sensible default sizes (and thus also a sensible minimal size) I don't think we should have a default size.