Document that Settings Sync do not sync remote extensions

[sandy081]

Document that Settings Sync do not sync remote extensions

[thernstig]

Add a note also to https://code.visualstudio.com/docs/editor/profiles about this, as well as https://code.visualstudio.com/docs/editor/extension-marketplace.

Adding it to the Profiles page is important since it goes at lengths describing how good Profiles is for extensions.

[sandy081]

Adding to Profiles page makes sense but not in marketplace page.