bug#1184: document how to deal with beer belly rectangles

[jidanni]

In (info "(emacs)Rectangles"), please
mention how to deal with this:

This paragraph you are reading is a
potential rectangle that is hard to put
point and mark around, because it is fat in
the middle. Thus while it is easy to mark its
left side, how are you going to mark its
right upper or lower corner without
altering the buffer to add spaces
in order to get the cursor there?
Worse yet, the buffer might
even be read-only.