Fix documentation for updated subpacakge test/example enable logic (#268)

[bartlettroscoe]

I had forgotten to update some documentation for how this impacted the tribits_add_[executable_and_][test|advanced_test]() functions.

This makes this documentation consistent with:

• 12. Enable/disable of parent package tests/examples is enable/disable for subpackages tests/examples
• 20. <Project>_ENABLE_TESTS only enables explicitly enabled package tests