The following features are almost not documented (AFAIK). semaphore locking with down() up() functions and the usage of them. The bit operations set_bit() clear_bit() and test_bit() are also missing usage information. Those functions are important for drivers programmers that should take care about SMP/resource locking. Please email me <firstname.lastname@example.org> refrences if you know about.
The KHG is missing an example section. Each function in the Linux kernel should have an example page in the KGH.