[DOC] Document the usage clearly about update operation with ingest pipeline

[gaobinlong]

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.

The update document API doesn't support pipeline parameter, which indicates that update operation doesn't support ingest pipeline, but if there's a default ingest pipeline or final ingest pipeline in the index, then that ingest pipeline can be executed sometimes, we need to document the usage clearly, do not confuse users.

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

2.19.0

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

opensearch-project/OpenSearch#16663

opensearch-project/OpenSearch#16712

[gaobinlong]

Please assign this to me, thanks!

[Naarcha-AWS]

@gaobinlong: Feel free to ping me once you have a PR open for this.