Docs: Add winter wheat info and instructions

### What sort(s) of documentation issue is this?
- [x] 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- [x] Something is (or might be) incorrect or outdated.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](https://escomp.github.io/CTSM/tech_note/index.html) (science and design of the model)
- [x] [User's Guide](https://escomp.github.io/CTSM/users_guide/index.html) (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
Winter wheat is mentioned in at least [one table](https://escomp.github.io/CTSM/tech_note/Plant_Hydraulics/CLM50_Tech_Note_Plant_Hydraulics.html#id12) but elsewhere are only mentioned [here](https://escomp.github.io/CTSM/tech_note/Ecosystem/CLM50_Tech_Note_Ecosystem.html): "Winter cereals (such as winter wheat) may be introduced in a future version of the model." Both scientific and usage information should be added.