[MNGSITE-494] Clarify how to modify inheritance with attributes prefixed with "combine."

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MNGSITE-494](https://issues.apache.org/jira/browse/MNGSITE-494?redirect=false)** and commented

The merging options used for inheritance are only partially described within https://maven.apache.org/pom.html#advanced_configuration_inheritance but in fact apply everywhere (not only to plugin configuration). Also the attributes `combine.id` and `combine.keys` used in https://github.com/codehaus-plexus/plexus-utils/blob/7f5114af7d0c4d19838a9e0082758ec0f5e0269e/src/main/java/org/codehaus/plexus/util/xml/Xpp3DomUtils.java#L124 are not described at all.

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No further details from [MNGSITE-494](https://issues.apache.org/jira/browse/MNGSITE-494?redirect=false)

Review the advanced_configuration_inheritance section at https://maven.apache.org/pom.html#advanced_configuration_inheritance and compare it with the inheritance behavior referenced in plexus-utils' Xpp3DomUtils.java. Document that the merging options apply beyond plugin configuration, and explain the combine.id and combine.keys attributes. Done means the relevant inheritance rules and both attributes are described.

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