XFree86 prior to version 4.2.0 comes with only the GLX and (maybe) GLU man pages. If you wanted a full set of OpenGL man pages, you had to get them yourself.
manglx.tar.Z (you don't need this one)
If you are using XFree86 4.2.0 or later, you should already have the manglx OpenGL interface extension man pages, mangl OpenGL library man pages, and the manglu GL Utility library pages too. If you are using XFree86 prior to 4.2.0, you'll have to see what you have included with your XFree86 version and get some of the packages above. As time goes on, XFree86 has been including more of these manual pages and other standard OpenGL add-on libraries.
These packages can be found at: ftp://ftp.sgi.com/sgi/opengl/doc/
These man page packages are in a format ready to be unpacked into the XFree86 source distributions (if needed/missing), see below.