Amended 9patch border documentation

<p>

    The border is used to define the stretchable and static areas of 

    the image. You indicate a stretchable section by drawing one (or more) 1-pixel-wide 

    black line(s) in the left and top part of the border. (You can have as 

    many stretchable sections as you want.) The relative size of the stretchable 

    sections stays the same, so the largest sections always remain the largest.

    black line(s) in the left and top part of the border (the other border pixels should

    be fully transparent or white). You can have as many stretchable sections as you want:

    their relative size stays the same, so the largest sections always remain the largest.

</p>

<p>

    You can also define an optional drawable section of the image (effectively,