doc: add NO_COLOR to disable colors in assert doc

[shobhitchittora]

Currently the assert docs point to only NODE_DISABLE_COLORS env variable to disable tty colors. NO_COLOR does the same thing.

Going ahead and creating a small PR to add the variable there.

• Version: v12.8.0
• Platform: Darwin Kernel Version 18.7.0
• Subsystem: doc

[lpinca]

Fixed by #30483.