Clarify docs for pytest.main default behavior

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

doc/en/how-to/usage.rst

@@ -173,7 +173,9 @@ You can invoke ``pytest`` from Python code directly:
 
 this acts as if you would call "pytest" from the command line.
 It will not raise :class:`SystemExit` but return the :ref:`exit code <exit-codes>` instead.
-You can pass in options and arguments:
+Be careful though, by default (when ``None`` is passed) ``main`` will read arguments
+from the command line, which may not be desirable.
+You can pass in options and arguments explicitly:
 
 .. code-block:: python
 

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 command line.
     :param plugins: List of plugin objects to be auto-registered during initialization.
 
     :returns: An exit code.