User's Guide: Force h2i in GDD-Generating workflow

(Step 4 in parent issue #3319. Only necessary if Step 1, #3320, indicates this is a good idea.)

### 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- [x] Something is (or might be) incorrect or outdated.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?
- [ ] [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

I think it would be better to request instantaneous `GDDACCUM` and `GDDHARV` in the [GDD-generating workflow](https://escomp.github.io/CTSM/users_guide/running-special-cases/Running-with-custom-crop-calendars.html#generating-maturity-requirements). See #3319 for more details.

See #3332 for additional updates that could happen when addressing this.