Change $slop docs and tester

[coryrc]

Incorporating findings from Issue #1679

Describing why based on the diff on my screen, starting top to bottom:

Removing XY vs Z axis, because the printing direction isn't known for the parts. Even precise CNC machining still has tolerances and I tried to write a simple way of passing that along.

Adding the limitations we talked about in issue #1679.

Added some more written examples of where slop should be added. Now that I'm writing this, these would be better as an example.

I can't add a new line to the first "Calibration" line because that'll make a new number. I thought this was good info to add though.

Moved caveat to Limitations section. Added line about filleting/chamfers now that I made the tester cut off corners, so you can test both ways.

Also fix a typo later in doc