Documentation: fix locking rt-mutex doc refs

This document tries to describe the design of the rtmutex.c implementation.

It doesn't describe the reasons why rtmutex.c exists. For that please see

Documentation/rt-mutex.txt.  Although this document does explain problems

Documentation/locking/rt-mutex.txt.  Although this document does explain problems

that happen without this code, but that is in the concept to understand

what the code actually is doing.

robust-futex, PI-futex, robust+PI-futex.

More details about priority inheritance can be found in

Documentation/rt-mutex.txt.

Documentation/locking/rt-mutex.txt.