Clarify what the "WPILib: Check for WPILib Updates" command actually updates

[calcmogul]

When it's run outside a robot project it does nothing. Someone on the FRC Discord thought it would do the same thing the latest installer does (thus making the installer redundant), when it apparently just updates the GradleRIO version in an open robot project.

[PeterJohnson]

Maybe it should have a detection for the system-installed version and pop up a download page for the installer if a newer one is available?

[sciencewhiz]

This is what frc-docs says https://docs.wpilib.org/en/stable/docs/software/vscode-overview/wpilib-commands-vscode.html

WPILib: Check for WPILib Updates - Check for an update to the WPILib GradleRIO version for the project. This does not update the Visual Studio Code extension, tools, or offline dependencies. Users are strongly recommended to use the offline wpilib installer