Removed rectangle example from co-ordinate system documentation

[radarhere]

#7155 (comment)

In the docs, it says, for the Coordinate System:

For example, a rectangle covering all of an 800x600 pixel image is written as (0, 0, 800, 600).

Is this not incorrect, as since the images are zero-based, that text ((0, 0, 800, 600)) should actually be for an image 801 x 601? And so that line of text should really be:

For example, a rectangle covering all of an 800x600 pixel image is written as (0, 0, 799, 599).

My first attempt to address this was #7156, but while ImageDraw's rectangle() includes endpoints, it was pointed out that crop() does not.

In this PR, I simplify things by removing the description of how a rectangle will relate to an image.