Update comments for v9 reference docs

[Feiyang1]

• Add package description for AppCheck
• Update links in Auth comments. It's not possible to link to a property of a variable, so we just link to the variable.
• Remove unnecessary package specifiers in links

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7aea39a

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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[github-actions]

Changeset File Check ⚠️

• Changeset formatting error in following file:

  Package "firebase-compat-typings-test" must depend on the current version of "firebase-exp": "9.0.0-beta.6" vs "file:../../packages-exp/firebase-exp"
  Package "firebase-compat-typings-test" must depend on the current version of "firebase-exp": "9.0.0-beta.6" vs "file:../../packages-exp/firebase-exp"
  Package "firebase-compat-typings-test" must depend on the current version of "firebase-exp": "9.0.0-beta.6" vs "file:../../packages-exp/firebase-exp"
  Error: Failed to find where HEAD diverged from master. Does master exist?
      at async getChangedFilesSince (/home/runner/work/firebase-js-sdk/firebase-js-sdk/node_modules/@changesets/git/dist/git.cjs.dev.js:169:22)
      at async Object.getChangedPackagesSinceRef (/home/runner/work/firebase-js-sdk/firebase-js-sdk/node_modules/@changesets/git/dist/git.cjs.dev.js:208:24)
      at async getStatus (/home/runner/work/firebase-js-sdk/firebase-js-sdk/node_modules/@changesets/cli/dist/cli.cjs.dev.js:977:27)
      at async run$1 (/home/runner/work/firebase-js-sdk/firebase-js-sdk/node_modules/@changesets/cli/dist/cli.cjs.dev.js:1227:11)
  

[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (99414a5) and head commit (c93d241).

Test Logs

• Base (99414a51): https://github.com/firebase/firebase-js-sdk/actions/runs/994796804
• Head (c93d2416): https://github.com/firebase/firebase-js-sdk/actions/runs/995063263

[google-oss-bot]

Size Analysis Report

Affected Products

Diffs between base commit (99414a5) and head commit (c93d241) are too large (517,485 characters) to display.

Please check below links to see details from the original test log.

Test Logs

• Head (c93d241): https://github.com/firebase/firebase-js-sdk/actions/runs/995063263

[egilmorez]

LG, thanks!