[MNG-6973] Document certain user properties in "mvn --help"

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MNG-6973](https://issues.apache.org/jira/browse/MNG-6973?redirect=false)** and commented

There are some user properties which can be used to influence behaviour of Maven which is not described via `mvn --help`:

1. `maven.repo.local` (https://github.com/apache/maven/blame/5739b12aa9fc03bdd9eb6482b7a75f98a3c1ba6e/maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java#L1603)
2. `style.color` https://github.com/apache/maven/blame/5739b12aa9fc03bdd9eb6482b7a75f98a3c1ba6e/maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java#L503

I haven't found any documentation around those anywhere but I consider both useful enough to be documented with `--help`

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**Affects:** 3.6.3