[docs] clarify the proper 'make-mode' usage in the docs (#11992)

doc/man/sphinx-build.rst

@@ -39,7 +39,11 @@ Options
  Extensions can add their own builders.
 
  .. important::
- Sphinx only recognizes the ``-M`` option if it is used first.
+ Sphinx only recognizes the ``-M`` option if it is used first, along with
+ the source and output directories, before any other options are passed.
+ For example::
+
+ sphinx-build -M html ./source ./build -W --keep-going
 
  The *make-mode* provides the same build functionality as
  a default :ref:`Makefile or Make.bat <makefile_options>`,