Doc: Add documentation about updating referenced auxiliary files.

[e-cite]

Make sure these boxes are signed before submitting your Pull Request -- thank you.

• [X] I have read the contributing guide lines at https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing
• [X] I have signed the Open Information Security Foundation contribution agreement at https://suricata-ids.org/about/contribution-agreement/
• [X] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)

Link to redmine ticket: https://redmine.openinfosecfoundation.org/issues/5376

Describe changes:

• Add a hint in the documentation about how Suricata-Update handles referenced auxiliary files

Thanks for feedback or merging :)