Documentation: relate width/height to position aesthetics

[teunbrand]

This issue is a proposal to include a little bit more details on how the x and width pairs and y and height pairs of aesthetics interact. I've seen enough confusion on this topic that I think this warrants a little section in ?aes_position.
Should basically point out:

• x and y are point-based locations
• width and height are length-based dimensions
• location-based aesthetics have scales and respond to transformation
• length-based aesthetics do not have scales and are applied after transformation
• Give concrete example that x = 10, width = 2 equates to xmin = 9, xmax = 11 under identity scales, but to xmin = 1, xmax = 100 under log10-scales.