Here is a list of all documented files with brief descriptions:

bigreq.h
composite.h
config.h
damage.h
dpms.h
dri2.h
dri3.h
glx.h
present.h
randr.h
record.h
render.h
res.h
screensaver.h
shape.h
shm.h
sync.h
xc_misc.h
xcb.h
xcb_windefs.h
xcbext.h
xcbint.h
xevie.h
xf86dri.h
xfixes.h
xinerama.h
xinput.h
xkb.h
xprint.h
xproto.h
xselinux.h
xtest.h
xv.h
xvmc.h