locking/spinlocks: Document the semantics of spin_is_locked()

There appeared to be a certain, recurrent uncertainty concerning the
semantics of spin_is_locked(), likely a consequence of the fact that
this semantics remains undocumented or that it has been historically
linked to the (likewise unclear) semantics of spin_unlock_wait().

A recent auditing [1] of the callers of the primitive confirmed that
none of them are relying on particular ordering guarantees; document
this semantics by adding a docbook header to spin_is_locked(). Also,
describe behaviors specific to certain CONFIG_SMP=n builds.

[1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2
    https://marc.info/?l=linux-kernel&m=152042843808540&w=2
    https://marc.info/?l=linux-kernel&m=152043346110262&w=2

Co-Developed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Co-Developed-by: Alan Stern <stern@rowland.harvard.edu>
Co-Developed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arch@vger.kernel.org
Cc: parri.andrea@gmail.com
Link: http://lkml.kernel.org/r/1526338889-7003-1-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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