Improve shard allocation awareness documentation - part 2

[patelsmit32123]

Description

Shard allocation awareness attributes documentation was lacking information regarding how the allocation actually functions and in what conditions the shard can remain unallocated.

Issues Resolved

Closes #[insert issue number]

Version

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

Frontend features

If you're submitting documentation for an OpenSearch Dashboards feature, add a video that shows how a user will interact with the UI step by step. A voiceover is optional.

Checklist

• [ ] By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

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[patelsmit32123]

@Naarcha-AWS I wanted to suggest some more changes w.r.t to the last PR. Please take a look.