[MNGSITE-445] Document "inlined collections" approach for plugin configuration

**[Andreas Sewe](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=asewe)** opened **[MNGSITE-445](https://issues.apache.org/jira/browse/MNGSITE-445?redirect=false)** and commented

I have searched in various places ([Guide To Configuring Plug-ins](https://maven.apache.org/guides/mini/guide-configuring-plugins.html), the [Your First Mojo tutorial](https://maven.apache.org/guides/plugin/guide-java-plugin-development.html#parameters), the [Mojo API documentation](https://maven.apache.org/developers/mojo-api-specification.html)) but couldn't find any mention of a feature introduced way back with Maven 3.0.3 and (AFAICT) only described in this blog post from 2011, [Configuring Plugin Goals in Maven 3](https://blog.sonatype.com/2011/03/configuring-plugin-goals-in-maven-3/): Inlined collections.

IMHO, this feature and also the other improvements mentioned in said blog post are currently missing from Maven's documentation.

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**Issue Links:**
- [MNG-7133](https://issues.apache.org/jira/browse/MNG-7133) Allow to inline collection/array items within plugin root configuration

Start with the Guide To Configuring Plug-ins, Your First Mojo tutorial, and Mojo API documentation, then compare them with the linked blog post about Maven 3 plugin goals. Check MNG-7133 for the inlined collection details; done means the missing approach and the other identified improvements are documented in the relevant Maven guides.

Written by the indexing model from the issue text.