[DOC] Update the document of the semantic field for a new parameter skip_existing_embedding

[bzhangam]

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.
Update the document of the semantic field for a new parameter skip_existing_embedding

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
3.2

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