The purpose of the PCT_*_MAX fields on the flanduse_timeseries file needs to be described

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- [X] Something is (or might be) incorrect or outdated.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

The purpose of the PCT_*_MAX fields on the flanduse_timeseries file needs to be described somewhere, probably in Chapter 2.28. Transient Land Use and Land Cover Change, as part of #3874 . These fields are:
PCT_URBAN_MAX
PCT_LAKE_MAX
PCT_CROP_MAX
PCT_NAT_PFT_MAX
PCT_CFT_MAX

The current version of the technical note reads:

"There is a special case when a given crop column type newly comes into existence - for example, when temperate corn first comes into existence in a gridcell. In this case, the column's below-ground temperature and water states are copied from the natural vegetated column in its gridcell, so that these state variables begin in a close-to-spun-up state. Other state variables (most of which spin up relatively quickly) begin at their cold start initialization values. This initialization is not necessary for the two other land unit types that currently can grow - natural vegetation and glacier: Those land unit types are always active, even when they have zero area on the gridcell, so their state variables will be spun up immediately when they come into existence. After this initialization, the conservation code described above takes effect."

But I think that only the natural vegetation pfts, crop pfts, urban landunits, and lake landunits that are identified by the PCT_*_MAX values that are greater than zero are always active, even when they have zero area on the gridcell? I'm not sure what is done with glacier landunits, maybe those are always active.