Add some extra emphasis on the breaking change in the helm parameters related to the allocator service

[roberthbailey]

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug
/kind cleanup

/kind documentation

/kind feature
/kind hotfix

What this PR does / Why we need it: Make the release note for 1.18-rc clearer and add an extra warning to the allocator service page in case folks didn't read the release notes.

Which issue(s) this PR fixes:

Closes #

Special notes for your reviewer:

[pooneh-m]

I don't think this note is needed as the document is versioned.

[roberthbailey]

That is true, and that's why I didn't originally add it. However, I think that if someone is going to look at this section it might not be immediately obvious what changed (unless you diff the pages or have really good memory). It's there as an extra bit of a reminder, and in case you didn't carefully read the breaking change in the release notes.

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pooneh-m, roberthbailey

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[google-oss-robot]

New changes are detected. LGTM label has been removed.

[agones-bot]

Build Failed 😱

Build Id: 780efb01-2df7-4151-994a-5e1ff696b20a

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Build Succeeded 👏

Build Id: 189b447e-50bd-4a69-82af-168b0c7cc419

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.18.0-8b7f0fe
• image: gcr.io/agones-images/agones-ping:1.18.0-8b7f0fe
• Linux C++ SDK (build): agonessdk-1.18.0-8b7f0fe-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.18.0-8b7f0fe.zip

A preview of the website (the last 30 builds are retained):

• https://8b7f0fe-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2305/head:pr_2305 && git checkout pr_2305
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.18.0-8b7f0fe