Add note about VSCodium update API to "Getting all the Telemetry Out"

[TyMick]

Not too long after switching from VS Code, I saw a software update available within VSCodium and downloaded and applied the update without thinking. Later, I was perusing Getting all the Telemetry Out, learned about the @tag:usesOnlineServices settings filter, tried using the filter in my own settings, and saw in the description for the update.mode setting, "The updates are fetched from a Microsoft online service."

That very much confused me, since I thought it meant that when I had used the internal software update function in VSCodium it had actually fetched those updates from Microsoft instead of from this repository. I remained confused until I happened upon #533, which led me to the update-api repo, which pointed me to #41 and the Auto Update Support project, and I finally understood.

So this PR is a little addition to "Getting all the Telemetry Out" that explains the update API in light of the update.mode setting description to hopefully prevent further confusion.

Here's the new text, starting with a new subheading within the "Getting all the Telemetry Out" section:

VSCodium Update API

When searching the @tag:usesOnlineServices filter, note that while the "Update: Mode" setting description still says "The updates are fetched from a Microsoft online service", VSCodium's build script sets the updateUrl field in product.json to that of VSCodium's own small update server, so enabling that setting won't actually result in any calls to Microsoft servers.

[stripedpajamas]

awesome, thank you!!