[MBUILDCACHE-120] Explain multiModule.discovery.scanProfiles

**[Gili](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=cowwoc)** opened **[MBUILDCACHE-120](https://issues.apache.org/jira/browse/MBUILDCACHE-120?redirect=false)** and commented

https://maven.apache.org/extensions/maven-build-cache-extension/build-cache-config.html defines `multiModule.discovery` as a section used to "specify how to discover multi module projects" and `scanProfile` as "Profiles to discover multi module project".

It's not clear what this means. What does the plugin do by default if this is not specified? What happens if users specify a list of profiles here?

Please provide more details and/or examples to help us understand what you mean here. Thank you.

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No further details from [MBUILDCACHE-120](https://issues.apache.org/jira/browse/MBUILDCACHE-120?redirect=false)

Start with the linked build-cache-config.html page, especially the multiModule.discovery and scanProfile descriptions. Trace the documented behavior of the Maven build cache extension for an omitted setting and for a supplied profile list, then update the page with clear explanations and examples covering both cases.

Written by the indexing model from the issue text.