Reword docs slightly

Author: nicoddemus

doc/en/customize.rst

@@ -38,8 +38,9 @@ Here's a summary what ``pytest`` uses ``rootdir`` for:
 Important to emphasize that ``rootdir`` is **NOT** used to modify ``sys.path``/``PYTHONPATH`` or
 influence how modules are imported. See :ref:`pythonpath` for more details.
 
-``--rootdir=path`` command line option sets a ``rootdir`` directory. Directory may be relative or absolute path.
-Additionally path may contain environment variables, that will be expanded.
+``--rootdir=path`` command-line option can be used to force a specific directory. 
+The directory passed may contain environment variables when it is used in conjunction
+with ``addopts`` in a ``pytest.ini`` file.
 
 Finding the ``rootdir``
 ~~~~~~~~~~~~~~~~~~~~~~~