chore(Themes): Add theme variable use notes

What does this MR do?

This adds a few notes in the theme color variables to explain why some themes don't have both light and dark variables. See gitlab-org/gitlab-services/design.gitlab.com#903 (comment 625119436) and gitlab-org/gitlab-services/design.gitlab.com!2460 (merged) for more info.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.

Security

N/A

Accessibility

N/A