[DOC] Improve Timechart command docs.

[penghuo]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.
https://github.com/opensearch-project/sql/pull/4379/files#diff-83ee41065c7b0dce55be1ee046ec533f8d3cc58f3e74c1fde096caaa7db6b995

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
OpenSearch 3.3

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.