Documentation for this library can be found in the included man pages, and in the Xlib spec from the specs subdirectory, also available at:
https://www.x.org/releases/current/doc/libX11/libX11/libX11.html https://www.x.org/releases/current/doc/libX11/libX11/libX11.pdf
and the O'Reilly Xlib books, which they have made freely available online, though only for older versions of X11:
X Series Volume 2: Xlib Reference Manual (1989, covers X11R3) https://www.archive.org/details/xlibretmanver1102nyemiss
X Series Volume 2: Xlib Reference Manual, 2nd Edition (1990, covers X11R4) https://www.archive.org/details/xlibrefmanv115ed02nyemiss
All questions regarding this software should be directed at the Xorg mailing list:
https://lists.x.org/mailman/listinfo/xorg
The master development code repository can be found at:
https://gitlab.freedesktop.org/xorg/lib/libX11
Please submit bug reports and requests to merge patches there.
For patch submission instructions, see:
https://www.x.org/wiki/Development/Documentation/SubmittingPatches