Update documentation for STL files

[RemiLehe]

Summary

Additional background

The documentation mentions the parameter eb2.stl_file. However, this parameter is ignored by AMReX unless one sets eb2.geom_type = stl (which is not mentioned in the current version of the documentation).

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate