[MSITE-726] Document how report sets are selected

**[Oliver B. Fischer](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=o.b.fischer)** opened **[MSITE-726](https://issues.apache.org/jira/browse/MSITE-726?redirect=false)** and commented

Set section [Configuring Reports](http://maven.apache.org/plugins/maven-site-plugin/examples/configuring-reports.html) of the Maven Site plugin describes how to configure multiple report sets.

After reading this section, I got the impression that I can define as many report sets as I would like to have. But the `aggregate` report set seems to be chosen for packing type `pom`.

Hence it would be very helpfull to have very simple and clear explanation how many reports are possible and which report set is chosen under which condition.

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**Affects:** 3.4

Start with the Maven Site plugin's Configuring Reports section, especially its description of multiple report sets and the aggregate report set for pom packaging. Document how many report sets are supported and which set is selected under each condition, with wording that resolves the reported ambiguity.

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