Add documentation about python -m pytest invocation.

changelog/911.doc

@@ -0,0 +1 @@
+Add documentation about the ``python -m pytest`` invocation adding the current directory to sys.path.

doc/en/pythonpath.rst

@@ -68,4 +68,9 @@ imported in the global import namespace.
 
 This is also discussed in details in :ref:`test discovery`.
 
+Invoking ``pytest`` versus ``python -m pytest``
+-----------------------------------------------
 
+Running pytest with ``python -m pytest [...]`` instead of ``pytest [...]`` yields nearly
+equivalent behaviour, except that the former call will add the current directory to ``sys.path``.
+See also :ref:`cmdline`.

doc/en/usage.rst

@@ -17,7 +17,7 @@ You can invoke testing through the Python interpreter from the command line::
     python -m pytest [...]
 
 This is almost equivalent to invoking the command line script ``pytest [...]``
-directly, except that Python will also add the current directory to ``sys.path``.
+directly, except that calling via ``python`` will also add the current directory to ``sys.path``.
 
 Possible exit codes
 --------------------------------------------------------------