Add documentation reference for chatops feature flag commands

What does this MR do?

All the examples in the feature flag documentation assume that you know what feature flags exist and that you want to change values. This change adds a reference to the file where the feature flag sub-commands are defined.

This also adds a paragraph to help a user determine if they are allowed to run feature flags at all.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer