[DOC] [Concurrent Segment Search] Experimental Release in 2.10

[sohami]

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.
In 2.10, we want to announce concurrent segment search as an experimental feature. For that we will need to add documentation for the users. This issue is to track the documentation related changes for experimental release.

[kolchfa-aws]

This issue is for the 2.10 experimental release.