Here is another way. Not really for Xcode, but I think it is still useful.
You can clone the repository with the source code of all OpenGL documents. Use your favorite SVN or CVS client:
svn co --username anonymous --password anonymous https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/docs/manglsl glsl svn co --username anonymous --password anonymous https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/docs/man2 opengl_2 svn co --username anonymous --password anonymous https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/docs/man3 opengl_3 svn co --username anonymous --password anonymous https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/docs/man4 opengl_4 svn co --username anonymous --password anonymous https://cvs.khronos.org/svn/repos/ogles/trunk/sdk/docs opengl_es_2_3 svn co --username anonymous --password anonymous https://cvs.khronos.org/svn/repos/registry/trunk/public/egl/sdk/docs opengl_es_1
After that, you will need to find the corresponding index.php file and open it in a browser. Thus, you will get manual pages with an index on the left side, as in offline documents.
You can grab more if you want: there are at least Super Bible books in the ecosystem repository. Just go up a few levels.
See the wiki page for more details: http://www.opengl.org/wiki/Getting_started/XML_Toolchain_and_Man_Pages
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