CN allocation tech notes improvements

[huiqi-wang]

Description of changes

Specific notes

Contributors other than yourself, if any:

• (Replace this text and add more list items as needed)

CTSM issues resolved or otherwise addressed, if any:

• references related:
  Line 14, 16: Plant Respiration → Chapter :numref:rst_Plant RespirationLine 14: Sprugel → :ref:Sprugel et al. (1995)
  Line 72: Larcher → :ref:Larcher (1995) Line 81: Allen / Vanninen / Axelsson — all three are now :ref: Lines 72, 161: Table → :numref:Table Allocation and CN ratio parameters
  Lines 163, 182: Equations → :eq:19.7, :eq:19.9, :eq:19.10, :eq:19.11, :eq:19.12 Lines 259, 335: FUN → Chapter :numref:rst_FUN

• Bug fixes:
  Line 72: g_l → g_1 (typo)
  Line 79: parentheses fixed in the a_3 sigmoid → \frac{2.7}{1 + e^{-0.004 (NPP_{ann} - 300)}} - 0.4
  Line 111: missing table row separator inserted (BDS boreal / C₃ arctic grass)
  Line 194: CF_{alloc,leaf} → CF_{alloc,leaf_tot} (consistent with Eq. 19.14–19.25)

• add more explanation and clarification:
  Line 45: 86400 is now explained (s day⁻¹, converts τ from days to s)
  Line 147: PFT abbreviation legend (NET/NDT/BET/BDT/BES/BDS + R/I)
  Lines 64–70 & 154–159: Eqs. 19.7 and 19.9 use \begin{aligned} + \text{}
  Line 256: section title capitalized "Nitrogen Allocation"
  Line 85: table title "carbon:nitrogen" (not :)
  Line 88: Target :math:CN_{...}`` (text outside math) — keep it consistant with similar item in CN Pools

If answers are expected to change, describe (delete this line otherwise):

• flag some but have not solved yet

1. Eq. 19.26 (min(demand, supply)) and Eq. 19.40 (N_uptake · demand_i / demand_tot) both define NF_{alloc,i}. This is confusing to me. I feel like there some sentences missing here, such as when When N_{uptake} ≥ NF_{demand,tot}, then xxxx, need help from science team.
2. The table shows 9-column in rst file but shows 7 columns in html. It may truncate in PDF; verify after a new html build, then decide split/landscape if needed

Any user interface changes (namelist or namelist defaults changes)?

Testing planned or performed, if any:

• (Replace this text and add more list items as needed)

Requirements before merge:

• I have followed the CTSM contribution guidelines.

• The code in this PR branch builds with no errors.
• The code in this PR branch runs with no errors. Briefly describe tested configuration(s):
• This either (a) does not change answers, (b) it only changes answers at roundoff level, or (c) I have performed a scientific evaluation of the answer changes. Which?:

• I have reviewed relevant parts of the CLM documentation Tech Note or User's Guide to determine if anything needs to be changed or added. If it does, describe:
• This PR either (a) does not create a need to update the documentation or (b) includes required documentation updates (see guidelines for contributing documentation). Which?:

[wwieder]

@slevis-lmwg do you mind reviewing this?