The kernel documentation can be found in the
linux-doc package. Once that is installed, you should be able to find the file under
/usr/src/linux/Documentation location is a bit of an anachronism. It used to be common to unpack the Linux kernel source code under
/usr/src/linux, which would have placed the documentation in the location you mentioned. However these days most people use the pre-built kernels from their distribution, and even if they do build their own kernel they are likely to do it in their home directory instead.