Documentation: Do a better job of saying which feature comes from Positron natively vs the Air extension

Positron native
 
- `Format Document`
- `Format Selection`
- Settings UI: Search for `format on save @lang:r`

Air extension

- `Air: Format Workspace Folder`

@mine-cetinkaya-rundel was looking for `> Format Document` in the command palette but was using a prefix of `> air:` and was confused when nothing relevant came up