Add section to developer's guide to make sure devs update FatesAllVars

[glemieux]

Is your feature request related to a problem? Please describe.
Developers need to make sure that the FatesAllVars testmod user_nl_clm in ctsm is updated whenever a new fates history variable is updated. Came up with in the context of triaging NGEET/fates#551.

Describe the solution you'd like
It should guide users through use of FindInactive.py to determine how to construct a full list of all variables. This probably should be added to https://fates-users-guide.readthedocs.io/en/latest/developer/How-to-add-a-new-FATES-history-variable.html.

Describe alternatives you've considered
No alternatives considered.

Additional context
None