Add more comments to BraveShieldsContentSettingSetFunction

[simonhong]

About why it handles js settings differently than others.

Close: brave/brave-browser#2326
This supplements #439

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[bbondy]

@simonhong each PR should close an issue. I can't tell which issue this is meant to close. The one mentioned seems to be in 0.55.x.

[simonhong]

@bbondy Oh, I should use separate issue when PRs are merged separately. brave/brave-browser#2326 is created and pointed by this PR. Sorry!

[bbondy]

np thanks!