which directory is the Linux kernel documentation be found?

In which directory is the Linux kernel documentation be found?

Answer: /usr/src/linux/Documentation

Explanation:
See http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/usr.html
/usr/src/linux/Documentation Contains documentation that may be necessary in order to re-compile
a kernel. However, it also provides quite a lot of information about your Linux system in general as well. For those who wish to seek further information on the contents of this directory you may consult the /usr/src/linux/Documentation/00-INDEX file. Further, more detailed documentation may be found in /usr/src/linux/Documentation/Docbook.