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 sphinx-doc#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:
 

sphinx/templates/quickstart/make.bat.new_t

@@ -25,11 +25,11 @@ if errorlevel 9009 (
 	exit /b 1
 )
 
-%SPHINXBUILD% -M %1 %SOURCEDIR% %BUILDDIR% %SPHINXOPTS%
+%SPHINXBUILD% -M %1 %SOURCEDIR% %BUILDDIR% %SPHINXOPTS% %O%
 goto end
 
 :help
-%SPHINXBUILD% -M help %SOURCEDIR% %BUILDDIR% %SPHINXOPTS%
+%SPHINXBUILD% -M help %SOURCEDIR% %BUILDDIR% %SPHINXOPTS% %O%
 
 :end
 popd

sphinx/templates/quickstart/make.bat_t

@@ -9,7 +9,7 @@ if "%SPHINXBUILD%" == "" (
 )
 set BUILDDIR={{ rbuilddir }}
 set SOURCEDIR={{ rsrcdir }}
-set ALLSPHINXOPTS=-d %BUILDDIR%/doctrees %SPHINXOPTS% %SOURCEDIR%
+set ALLSPHINXOPTS=-d %BUILDDIR%/doctrees %SPHINXOPTS% %O% %SOURCEDIR%
 set I18NSPHINXOPTS=%SPHINXOPTS% %SOURCEDIR%
 if NOT "%PAPER%" == "" (
 	set ALLSPHINXOPTS=-D latex_elements.papersize=%PAPER%paper %ALLSPHINXOPTS%