Review 2.17. CN Pools; and update

[slevis-lmwg]

What sort(s) of documentation issue is this?

• Something is missing.
• Something is (or might be) incorrect or outdated.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 (science and design of the model)
• 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

See instructions in the parent issue.

Additional context (optional)

Add any other context or screenshots about the issue here.

[slevis-lmwg]

I see one piece of missing information in Chapter 2.17, Section 2.17.2: https://escomp.github.io/CTSM//tech_note/CN_Pools/CLM50_Tech_Note_CN_Pools.html#tissue-stoichiometry

I'm copying here from WhatsNewInCTSM5.4.md:

Potentially time-evolving `leafcn_target` replaces time-constant `leafcn`

The former is calculated as a function of the latter and can be time-evolving depending on new paramfile parameter `leafcn_co2_slope` #1654. The time-evolving effect defaults to off with `leafcn_co2_slope` = 0 on the parameter file.

[slevis-lmwg]

TODOs

• Links in this chapter currently work.
• Add text shown above to Section 2.17.2.
• Add the reference.
• Open PR.
• Check Table 2.17.1 against the paramfile that we're releasing:
  So far I have checked and updated against ctsm60_params.c260303.nc. I found miscanthus and switchgrass with leafcn = 20 in the file so I updated them in the table from 25. Or should we update the paramfile?
• Build the docs to confirm that this and other sections that I have modified have worked.