Document deprecation of support for system python on Mac

[DonJayamanne]

In #517 we'll be deprecating support for system python on Mac. This will need to be documented and properly communicated in the new release.

[brettcannon]

Docs should be generic and just say "don't use it" since the system install is just wonky/bad period, regardless of whether someone installed pip and virtualenv manually.

[brettcannon]

Fixed in microsoft/vscode-docs#1350 .