Document for instant/completion time clearly

[hudi-bot]

This is a followup ticket for: HUDI-8077

We should document instant/completion time usage for clearly, e.g., Cleaning, and Tech Spec pages, so that users know the start/instant time is still used in the clean metadata, if they want to use the timestamp for debugging.

JIRA info

• Link: https://issues.apache.org/jira/browse/HUDI-8258
• Type: Sub-task
• Parent: https://issues.apache.org/jira/browse/HUDI-8894
• Fix version(s):
  • 1.1.0