[MNG-6348] Clarify Maven Plugin Configuration Locations and their Order of Precendence

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MNG-6348](https://issues.apache.org/jira/browse/MNG-6348?redirect=false)** and commented

There are multiple locations in the pom where you can configure build plugins, namely
1. PluginManagement -> Global Configuration
2. PluginManagement -> Execution Configuration
3. Plugins -> Global Configuration
4. Plugins -> Execution Configuration

Those locations should all be listed in https://maven.apache.org/guides/mini/guide-configuring-plugins.html#Configuring_Build_Plugins and the order or precedence should be explicitly listed there.

Is the order given above correct at all, meaning that setting `param1` in 4. will overwrite the configuration of `param1` in 1., 2. or 3.?

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