Add %O% option to make.bat templates; update docs

The make.bat_t and make.bat.new_t templates now pass %O% as well as
%SPHINXOPTS% to `sphinx-build`, in parity with Makefile.new_t.

The sphinx-build documentation page was updated to explicitly note
$(O) and %O% as shortcut variables.

Closes #6323, per the comment thread discussion.

doc/man/sphinx-build.rst

@@ -302,7 +302,8 @@ variables to customize behavior:
 
 .. describe:: SPHINXOPTS
 
-   Additional options for :program:`sphinx-build`.
+   Additional options for :program:`sphinx-build`. These options can
+   also be set via the shortcut variable **O** (capital 'o').
 
 .. _when-deprecation-warnings-are-displayed: