docbook: make kernel-locking table readable

Andi Kleen pointed out to me that the kernel locking cheat sheet
table entries are unreadable.

Make table entries smaller so that pdf and ps output is readable
(columns were being overwritten and garbled) by using abbreviations.
This allows the tables to fit on one page cleanly.
Add a Legend for the abbreviations:
  SLIS: spin_lock_irqsave
  SLI:  spin_lock_irq
  SL:   spin_lock
  SLBH: spin_lock_bh
  DI:   down_interruptible

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>