Better docs for repo analysis failures

[DaveCTurner]

Today the docs for the repository analysis API say that failures are evidence of incompatibilities, but we do not go into much detail about the exact error messages returned by the API. We should address this to avoid users needing to open support cases to ask for help interpreting its results:

• Introduce specific docs for each error the API might return.
• Use org.elasticsearch.common.ReferenceDocs to add a link to the specific docs from each error message.
• Extend the "Implementation details" docs to mention register operations.
• Document a little more detail about the compare-and-exchange implementation for each repository type, particularly S3.

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)