Tech Note: Distinguish between time step and iteration level in vegetation longwave and fluxes

[olyson]

Description of changes

Distinguish between time step and iteration level in vegetation longwave and fluxes tech note documentation

Specific notes

Contributors other than yourself, if any: @bishtgautam

CTSM Issues Fixed (include github issue #):

• Resolves #3035

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so? Yes, yes.

Testing performed, if any: Built and view documentation locally.