[DOC] Default Model Id for Neural Search Query clause

[vibrantvarun]

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 and all versions that are affected.
With recent addition of a new feature customer can add a default model Id at field level as well as index level by creating a search pipeline for executing neural search.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/neural-search#70