docs: fix tidb_server_memory_limit unit description (release-8.4)

[vjymisal0]

What is changed, added or deleted? (Required)

• Fix the tidb_server_memory_limit unit description in system-variables.md on the release-8.4 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-8.4

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[vjymisal0]

Tide reports Merging to branch release-8.4 is forbidden — this release branch appears to be frozen/EOL for direct merges, independent of review status. Closing this PR since it can't land here; the content fix (matching the already-merged #23587/#23588/#23660) is still valid if a maintainer wants to cherry-pick it via the needs-cherry-pick-release-* bot flow from master instead.