[MNGSITE-495] Clarify merging of repositories/pluginRepositories

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MNGSITE-495](https://issues.apache.org/jira/browse/MNGSITE-495?redirect=false)** and commented

According to https://github.com/apache/maven/blob/2a9f39336cec1d8e52d30cc48503d51ed8672536/maven-model-builder/src/main/java/org/apache/maven/model/merge/MavenModelMerger.java#L263 repositories from child poms overwrite repositories with the same id from the parent pom completely (instead of merging them per id). As this is unexpected and different from plugin execution ids it should be clarified in https://maven.apache.org/guides/introduction/introduction-to-the-pom.html#Project_Inheritance

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No further details from [MNGSITE-495](https://issues.apache.org/jira/browse/MNGSITE-495?redirect=false)

Start with the Project Inheritance section at https://maven.apache.org/guides/introduction/introduction-to-the-pom.html#Project_Inheritance and inspect the cited MavenModelMerger.java location for the repository behavior. Clarify how repositories and pluginRepositories with matching IDs are handled during inheritance, then verify that the documentation accurately distinguishes this behavior from plugin execution IDs.

Written by the indexing model from the issue text.