Docs: Specify that snow/ice units are liquid water equivalent

[samsrabin]

Discussed in #2910

^{Originally posted by gunterl December 16, 2024}
This item came up when creating land model diagnostics. It would be great to specify somewhere in the documentation that units regarding snow and ice are outputted in water equivalent (compared to some model who provide output in ice equivalent).

Thanks a lot.

[samsrabin]

Shuyue Li has made and tested a local fix for this; PR forthcoming.