Documentation clarifying shape of bounds, adding tests to verify expected behaviour

When trying to create some coordinates on the fly I was a bit unclear on what the bounds should be - I've tried to clarify this in the API documentation.  
I've also tried to put in a few extra tests around this behaviour (I couldn't see them anywhere else - though apologies if I missed them, I'm still finding my way round things with limited time to explore in depth). 
I tightened up the test conditions a little - though whether what I've done is the right thing I'm not sure.