Add notice to users to distinguish differences between ELM and CLM

[glemieux]

Is your feature request related to a problem? Please describe.
NGEET/fates#1399

Describe the solution you'd like
Make sure that users understand that post CLM4.5, ELM and CLM are diverging and there should be no expectation that you can switch between the land models and get the same scientific assumptions.

Describe alternatives you've considered
None.

Additional context
This is largely assumed to not be an issue as modelers don't typically (?) work with different land models.