Document pom.xml plugin inheritance mechanism

[lukaseder]

It could be useful to document how Maven's plugin configuration inheritance works, in the manual:
https://maven.apache.org/pom.html#Plugins

For example, if more than 1 code generation run is defined, then declare:

<pluginManagement>
  ...
  <configuration>
    <generator>
      <database>
        <forcedTypes>
          ...

And in the actual plugin execution configurations, do:

<plugin>
  ...
  <configuration>
    <generator>
      <database>
        <forcedTypes combine.children="append">
          ...