Clarify that `update.channel` site config controls telemetry

Created by: DaedalusG

• Sourcegraph version: v3.x.x
• Platform information: any deployment type

Expected behavior:

Sourcegraph displays a banner to site admins when there is a newer version of Sourcegraph available. This banner is triggered when an update check pings Sourcegraph.com and determines that there is a new version.

Users who want to get rid of this banner may seek to prevent Sourcegraph from pining Sourcegraph.com to check for updates. They may then navigate to the site admins Update panel and find there the following interface --

This message needs to be clarified to warn admins that setting the site config option "update.channel"="none", will disable telemetry important for Sourcegraph business ops.

Suggested Solution:

• Add clarification to Update page in Sourcegraph instances
• Ammend the site config json schema and docs to include a warning that telemetry is also compiled by the update check goroutine

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