Miscellaneous improvements of the first two sections:

• Don't state that firewalls and stateful filtering will always improve security, since that might not be the case.
• Change the Firewall Rulesets section to a more general Firewall Concepts section.
• Remove some basic network text which the user should already know when reading about more advanced topics, like firewalls.
• Miscellaneous rewording.

Reviewed by: -doc
Improvments by: remko, David Adam <zanchey@ucc.gu.uwa.edu.au>