Update docs for feat: table/column/flow COMMENT

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What's Changed in this PR

Documents the COMMENT feature from GreptimeTeam/greptimedb#7060 for ALTER TABLE operations. The CREATE statements and INFORMATION_SCHEMA already documented COMMENT support; this adds the missing ALTER operations.

Changes

• ALTER TABLE syntax: Added MODIFY COLUMN name SET COMMENT comment for column-level comments
• Table options: Added comment option for table-level comments via SET/UNSET
• Examples: Setting/unsetting comments on tables and columns
• Metadata: Updated keywords to include "column comment" and "table comment"

Example Usage

-- Set column comment
ALTER TABLE monitor MODIFY COLUMN load_15 SET COMMENT 'Load average over 15 minutes';

-- Set table comment
ALTER TABLE monitor SET 'comment'='Monitor metrics table';

-- Remove table comment
ALTER TABLE monitor UNSET 'comment';

Comments are viewable via SHOW CREATE TABLE or INFORMATION_SCHEMA.COLUMNS/INFORMATION_SCHEMA.TABLES.

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update docs for feat: table/column/flow COMMENT</issue_title>
<issue_description>A document change request is generated from GreptimeTeam/greptimedb#7060</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update docs for feat: table/column/flow COMMENT #2166

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