Indicate why we have two variables for snow cover fraction (frac_sno_albedo and frac_sno_fluxes) and when they are different.

[olyson]

What sort(s) of documentation issue is this?

• Something is missing.
• Something is (or might be) incorrect or outdated.
• Something is confusing.
• Something is broken.

What part(s) of the documentation does this concern?

• Technical Note (science and design of the model)
• User's Guide (using the model and related tools)
• Somewhere else (e.g., README file, tool help text, or code comment): Please specify
• I don't know

Describe the issue

As part of PR #4185, the frac_sno and frac_sno_eff variables were renamed to frac_sno_albedo and frac_sno_fluxes to better distinguish the difference between these two variables.
@swensosc also recommends "It would be a good idea to have a more detailed comment somewhere indicating why we have two variables and when they are different, i.e. which landunits (I think non-natveg and non-crop), and why (albedo uses fractional value, but heat transfer does not for those landunits). "
Presumably this detailed comment would go somewhere in the tech note.