add debug/release tip to cpp docs (#17452)
Summary:
as title. These were already added to the tutorials, but I didn't add them to the cpp docs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17452
Differential Revision: D14206501
Pulled By: suo
fbshipit-source-id: 89b5c8aaac22d05381bc4a7ab60d0bb35e43f6f5
