Add a bit of additional information about running tests in a module for new users

Signed-off-by: Bryce Gattis <bryce.gattis@brazenanimation.com>

doc/en/how-to/usage.rst

@@ -26,6 +26,11 @@ Pytest supports several ways to run and select tests from the command-line or fr
 
     pytest test_mod.py
 
+When passing a path as a positional argument like this, it's not possible to
+pass a file basename. If you want to pass a path to a file, you must pass
+either a relative path or an absolute path. Otherwise, consider the below
+keyword expressions instead.
+
 **Run tests in a directory**
 
 .. code-block:: bash