docs(association): document uniqueKey

[sushantdhiman]

Pull Request check-list

Please make sure to review and check all of these items:

• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Have you added new tests to prevent regressions?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Did you update the typescript typings accordingly (if applicable)?
• Did you follow the commit message conventions explained in CONTRIBUTING.md?

Description of change

Fixes #1944

[codecov]

Codecov Report

Merging #12166 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12166   +/-   ##
=======================================
  Coverage   96.34%   96.34%           
=======================================
  Files          95       95           
  Lines        9203     9203           
=======================================
  Hits         8867     8867           
  Misses        336      336           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 55678d2...6f6837b. Read the comment docs.