[OSDOCS#17270]: Document dnsName allow vs 0.0.0.0/0 deny behavior

[harsh21nama]

Summary

• Documents that an IPv4 cidrSelector: 0.0.0.0/0 deny rule denies all egress traffic because connections are sent to IP addresses.
• Clarifies that an Allow rule with dnsName does not permit that traffic: after FQDN resolution, the connection uses the resolved IP and matches the deny-all CIDR rule.
• Shows the working pattern: allow specific destinations with cidrSelector, plus a note on matching order and the example CR.

Test plan

• Confirm the limitation, matching-order note, and DNS restriction read correctly on the configuring-egress-firewall-ovn preview.
• Confirm the IMPORTANT admonition appears next to the example EgressFirewall CR that uses cidrSelector + 0.0.0.0/0 deny.

Version(s):
main (in-development); cherry-pick to 4.18+ as needed

Issue:
https://redhat.atlassian.net/browse/OSDOCS-17270

Link to docs preview:
(add after preview build)

QE review:

• QE has approved this change.

[openshift-ci]

Hi @harsh21nama. Thanks for your PR.

I'm waiting for a openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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