UPDATING: Strengthen recommendations

Strengthen recommendations for ports_modules by adding a separate note.
Use the unused [2] to add a note about creating backups.

MFC after: 3 days
Reviewed by: brooks, mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D50126

(cherry picked from commit ca9131b729757959cb0fd4721603466284cfc6fe)