Better description of crown height and fraction in docs

[davidorme]

This is more related to the crown module, but when looking at the figure below containing the community crown area, it seems like crowns can run all the way to the ground. I would assume that each stem has a crown-free section. Is this something that is included in the model (for example by restricting the lower limit of z values depending on stem height)?

Posted by @arne-exe

Hmmm. This is something I need to clarify in the crown model. If I've got this right, the crown is technically only the upper surface of the overall crown shape (above the maximum crown radius). So the stem below that level, which has a relative height of 1 - the crown fraction, is crown free.

BUT... The crown gap fraction pushes leaf area down below that crown surface layer, so trees where the crown gap fraction is > 0 will have some leaf surface below that crown fraction height.

I might make this a new issue, just to get this closed down!

Originally posted by @davidorme in #503 (comment)