Seems like signs cannot be edited once you have added your text and clicked, OK? Am I wrong?
Point the cursor at the sign, press "F," and in the radial menu that appears, click "Edit."
As far as I know, in the future we should be able to select multiple plants using the F6 tool and "paint" them using Function 2.
However, even now, Function 2 already places multiple plants of the same type.
I expect that with the new biomes—which are still to come—new terrain types will arrive as well.