Add a warning about always updating the ports-base subcollection
whenever any part of the Ports Collection is updated to the CVSup
chapter of the Handbook.

Add a similar Q&A to the 'User Applications' section of the FAQ,
with the answer referencing the warning in the Handbook.

Reviewed by: keramida
Approved by: ceri (mentor), keramida, Stijn Hoop <stijn@win.tue.nl>