docs:Fix typos and resolve citation placeholder for CLM 4.0

[Labreo]

Summary

• Motivation: To improve readability by fixing typos and to address a documentation issue where a placeholder (TK- ref) was left in the Nitrogen Volatilization section regarding the Community Land Model (CLM) 4.0.
• What: Fix simple typos and replace the placeholder with reference to this paper: https://www.researchgate.net/publication/277114326_Technical_Description_of_version_40_of_the_Community_Land_Model_CLM which is the likely reference as it is the most popular paper i could find on community land model 4.0 that fit in the given placeholder context.Also credited the authors in the reference section below.I didn't want to link the paper directly in the documentation since they all had doi.org links.

How to test

Steps to reproduce and verify the change locally:

make
cd tests/smoke/<new-or-updated-test> ../../sipnet -i sipnet.in

Related issues

• Fixes # (or "Relates to #N" if this is not a resolution of that ticket)

Checklist

• Tests added for new features
• Documentation updated (if applicable)
• docs/CHANGELOG.md updated with noteworthy changes
• Code formatted with clang-format (run git clang-format if needed)
• Requested review from at least one CODEOWNER

For model structure changes:

• Removed \fraktur font formatting from docs/model-structure.md for implemented features

---

Note: See CONTRIBUTING.md for additional guidance. This repository uses automated formatting checks; if the pre-commit hook blocks your commit, run git clang-format to format staged changes.

[Alomir]

Looks good, thanks for the changes @Labreo!