[MINVOKER-270] Documentation error on postBuildScript explanations

**[Simon Althoff](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=simonalthoff)** opened **[MINVOKER-270](https://issues.apache.org/jira/browse/MINVOKER-270?redirect=false)** and commented

In the table of global variables in the [documentation](https://maven.apache.org/plugins/maven-invoker-plugin/examples/post-build-script.html) it says that 'baseDir' is 'The absolute path to the base directory of the test project.'

When running a script that is not located in the test projects root, 'baseDir' is set to the script's directory. This contradicts the documentation.

See relevant line in the [ScriptRunner](https://github.com/apache/maven-script-interpreter/blob/aec79dd1348b323ec1e23338908508eedf6142c6/src/main/java/org/apache/maven/shared/scriptinterpreter/ScriptRunner.java#L227)

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**Affects:** 3.2.1

**Issue Links:**
- [MINVOKER-320](https://issues.apache.org/jira/browse/MINVOKER-320) Placeholder basedir is confusing, has different value in filtering and scripts

Open the post-build-script documentation and compare its global-variable table with the behavior referenced in ScriptRunner.java around line 227. Update the baseDir explanation to match the behavior when a script is outside the test project root, and ensure the resulting wording does not conflict with the related MINVOKER-320 issue.

Written by the indexing model from the issue text.