The kernel provides a variety of locking primitives. The nesting of these
lock types and the implications of them on RT enabled kernels is nowhere
documented.

Add initial documentation.

Signed-off-by: Thomas Gleixner
Signed-off-by: Peter Zijlstra (Intel)
Link: https://lkml.kernel.org/r/20200321113242.026561244@linutronix.de