Add FLOAT4, FLOAT8 and MIDDLEINT to the docs

[dveeden]

What is changed, added or deleted? (Required)

Update the keyword docs

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v7.4 (TiDB 7.4 versions)
• v7.3 (TiDB 7.3 versions)
• v7.2 (TiDB 7.2 versions)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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

• This PR is translated from:

• Other reference link(s):

• parser: Add FLOAT4,FLOAT8 and MIDDLEINT compatibility types tidb#46649

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

[dveeden]

Note that there is a script to check keywords:

$ ./scripts/check-keywords.sh 
Missing docs for non-reserved keyword: ROLLUP
Missing docs for non-reserved keyword: HYPO

For this you need to make sure to check out the right branch of pingcap/tidb

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-06 07:58:41.331291103 +0000 UTC m=+2518685.880307090: ☑️ agreed by ran-huang.

[ran-huang]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ran-huang

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