/usr/share/man/man3/Tk_SetWindowBackground.3tk.gz is in tk8.6-doc 8.6.1-3ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/Tk_ConfigureWindow.3tk.gz
/usr/share/man/man3/Tk_ConfigureWindow.3tk.gz