$ man XRenderFillRectangle
No manual entry for XRenderFillRectangle
I already have thees packages installed: xorg-docs-core libx11-doc libx11-dev libxrender-dev
First you need someone to write man pages. libXrender currently doesn't have any man pages written, only the incomplete API doc at http://cgit.freedesktop.org/xorg/lib/libXrender/tree/doc/libXrender.txt.