docs: fix tidb_server_memory_limit unit description (release-8.1)

[vjymisal0]

What is changed, added or deleted? (Required)

• Fix the tidb_server_memory_limit unit description in system-variables.md on the release-8.1 branch, matching the wording already fixed on master (docs: add memory unit example for tidb_server_memory_limit #23587) and backported to release-7.5 (docs: fix tidb_server_memory_limit unit description #23588) and release-8.5 (docs: fix tidb_server_memory_limit unit description (release-8.5) #23660).
• Clarify that both KiB|MiB|GiB|TiB and KB|MB|GB|TB unit formats are supported, and add a 90GiB example.

Which TiDB version(s) do your changes apply to? (Required)

• release-8.1

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign jackysp for approval. For more information see the Code Review Process.
Please ensure that each of them provides their approval before proceeding.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 84fec06a-fdd6-43eb-b781-aaa589b8f10f

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[ti-chi-bot]

Hi @vjymisal0. Thanks for your PR.

I'm waiting for a pingcap 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.

I understand the commands that are listed here.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[pingcap-cla-assistant]

All committers have signed the CLA.