lockdep: Update documentation for lock-class leak detection

There are a number of bugs that can leak or overuse lock classes,
which can cause the maximum number of lock classes (currently 8191)
to be exceeded.  However, the documentation does not tell you how to
track down these problems.  This commit addresses this shortcoming.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>