Clarify docs for pytest.main default behavior (#11187)

Add explicit mentions of the default behavior to both API reference and
how-to section about ways to invoke pytest.

Co-authored-by: Ran Benita <ran@unusedvar.com>

src/_pytest/config/__init__.py

@@ -137,7 +137,9 @@ def main(
 ) -> Union[int, ExitCode]:
     """Perform an in-process test run.
 
-    :param args: List of command line arguments.
+    :param args:
+        List of command line arguments. If `None` or not given, defaults to reading
+        arguments directly from the process command line (:data:`sys.argv`).
     :param plugins: List of plugin objects to be auto-registered during initialization.
 
     :returns: An exit code.