[MINVOKER-320] Placeholder basedir is confusing, has different value in filtering and scripts

**[Slawomir Jaranowski](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=slawekjaranowski)** opened **[MINVOKER-320](https://issues.apache.org/jira/browse/MINVOKER-320?redirect=false)** and commented

When we use `basedir` for project file filtering - it points to the project base directory of the main build

But in scripts it is path to the base directory of the test project.

https://maven.apache.org/plugins/maven-invoker-plugin/examples/filtering.html

https://maven.apache.org/plugins/maven-invoker-plugin/examples/post-build-script.html

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**Issue Links:**
- [MINVOKER-270](https://issues.apache.org/jira/browse/MINVOKER-270) Documentation error on postBuildScript explanations

Compare the filtering and post-build-script examples linked in the issue, focusing on how each explains the basedir placeholder. Update the relevant documentation so the differing values are explicit and unambiguous, then verify that both examples consistently describe the intended behavior.

Written by the indexing model from the issue text.