Update best practices with note for `after` hook

What does this MR do?

Add a note to limit the use of after hook to non-UI operations only (Until we have https://gitlab.com/gitlab-org/quality/team-tasks/issues/270)

Related comment: #37308 (comment 250952155)

Conformity

• [-] Changelog entry
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content