Update Geo limitations

What does this MR do?

This MR updates several items in Geo limitations section in the documentation.

• Real-time updates of issues/merge requests does not work is now rescoped to only apply to secondary site's that have disabled HTTPS proxying. This limitation was applicable across the board before we make HTTPs proxying default behaviour. Now disable HTTPS proxying sites are the exception.
• Tweaked wording related to limitation related to DR on deployments that have multiple secondary sites need only to re-synchronization of PG data now. The other data will heal itself once PG replication is updated to follow the new primary.
• Refined limitation around SSH proxying to highlight the availability of new implementation for Linux-packaged deployments and added links to issue that tracks work needed for CNG.

Related issues

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