docs: fix tidb_server_memory_limit unit description (#23588)

[ti-chi-bot]

This is an automated cherry-pick of #23588

What is changed, added or deleted? (Required)

• Fix the tidb_server_memory_limit unit description in system-variables.md on the release-7.5 branch.
• Clarify that the supported memory-size format is written without spaces by using KB|MB|GB|TB, and add 90GB as an example.

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What is the related PR or file link(s)?

• This PR is translated from: docs: fix tidb_server_memory_limit unit description docs-cn#21894
• Other reference link(s): N/A

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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