Improve Pull Mirroring documentation

Review the functionality of Pull Mirroring, and the associated documentation.

Confirm if the feature currently offers:

• a configurable mirroring schedule option
• disabling the automatic intervals on a project level to allow for manual only syncs
• any differences for pull mirroring that apply to self-managed (eg: in the admin area only)

If these features currently exists, then update the documentation to explain how to do this. Otherwise create follow-up issues as typefeature