openstack: warn that overlays match by exact name

[ideaship]

What

Adds a warning admonition to the overlay section of the OpenStack configuration
guide: overlay files are matched by exact name and exact path, and a file
that matches none of the supported locations is silently ignored.

Why

The section explains which locations are searched, but not what happens when a
file does not match one of them. Nothing does — the file is not read, no warning
is emitted, and the deployment succeeds while the settings have no effect.

Operators therefore have no signal distinguishing "my override was applied" from
"my override was ignored", and the failure is easy to hit because near-miss
names look plausible.

How

Documents the two cases seen in the wild, both of which silently dropped the DNS
and MTU settings they carried:

• environments/kolla/files/overlays/ml2_conf.ini — ml2_conf.ini is only read
  from the neutron/ subdirectory, so a copy at the top level does nothing.
• environments/kolla/files/overlays/neutron/ml2.conf — a misspelling of
  ml2_conf.ini; nothing matches ml2.conf.

The admonition sits next to the existing description of the supported locations,
and suggests confirming the setting actually arrived by checking the generated
configuration under /etc/kolla/SERVICENAME/ rather than assuming the overlay
took effect.

Markdown-only addition to an existing page; yarn build has not been run
locally.

Related

Independent of, but found in the same investigation as:

• neutron: document DNS resolution for OVN #1041

🤖 Generated with Claude Code

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

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Detailed Issues

⚠️ SPELL / lychee - 1 error

📝 Summary
---------------------
🔍 Total..........880
🔗 Unique.........713
✅ Successful.....820
⏳ Timeouts........13
🔀 Redirected.......3
👻 Excluded........46
❓ Unknown..........0
🚫 Errors...........1
⛔ Unsupported......1

Errors in docs/appendix/security/ossa-2026-001.md
[TIMEOUT] https://bugs.launchpad.net/keystonemiddleware/+bug/2129018 (at 124:3) | Request timed out

Errors in docs/appendix/security/ossa-2026-002.md
[TIMEOUT] https://bugs.launchpad.net/nova/+bug/2137507 (at 111:3) | Request timed out

Errors in docs/appendix/security/ossa-2026-005.md
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2142138 (at 109:3) | Request timed out

Errors in docs/appendix/security/ossa-2026-015.md
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2148398 (at 174:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2148477 (at 175:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2149775 (at 176:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2149789 (at 177:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2150089 (at 178:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2150379 (at 179:3) | Request timed out

Errors in docs/appendix/security/ossa-2026-022.md
[TIMEOUT] https://bugs.launchpad.net/nova/+bug/2151252 (at 145:3) | Request timed out

Errors in docs/concepts/index.md
[ERROR] file://docs/concepts/metalbox#sonic-ztp (at 70:1) | File not found. Check if file exists and path is correct

Errors in docs/release-notes/osism-7.md
[TIMEOUT] https://www.openstack.org/software/openstack-bobcat (at 977:38) | Request timed out

Errors in docs/release-notes/osism-8.md
[TIMEOUT] https://www.openstack.org/software/openstack-caracal (at 222:38) | Request timed out

Errors in docs/release-notes/osism-9.md
[TIMEOUT] https://bugs.launchpad.net/kolla/+bug/2111620 (at 96:47) | Request timed out

Hint: Followed 3 redirects. You might want to consider replacing redirecting URLs with the resolved URLs. Use verbose mode (`-v`/`-vv`) to see redirection details.

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