Update substitutions in docs

[samsrabin]

What sort(s) of documentation issue is this?

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• [x] Something is (or might be) incorrect or outdated.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

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

• [x] Technical Note (science and design of the model)
• [x] 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

E.g., "CTSM1" for latest development code. See substitutions.py.