[PATCH doc] update documentation of x86 -mcx16 option

[Alexander Monakov <amonakov at ispras dot ru>]

Hi,

This patch fixes a few issues in documentation of -mcx16 x86 backend option:

- remove implementor-speak ('oword')
- mention alignment restriction and availability only in 64-bit mode
- improve usage example
    existing documentation uses a really silly example (128-bit integer
    counters), it was apparently taken from Wikipedia back in 2007; today,
    Wikipedia uses a far more realistic example ("This is useful for parallel
    algorithms that use compare and swap on data larger than the size of a
    pointer, common in lock-free and wait-free algorithms")
- mention that the instruction is used when expanding __sync builtins, and NOT
  used when expanding __atomic builtins. This is a quiet change in GCC-7, GCC-6
  and earlier used this instruction for __atomic builtins too.

OK for trunk?  For the gcc-7 branch?

Thanks.
Alexander

	* doc/invoke.texi (mcx16): Rewrite.

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 3308b63..0b3c296 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -25160,13 +25160,12 @@ This option instructs GCC to use 128-bit AVX instructions instead of

 @item -mcx16
 @opindex mcx16
-This option enables GCC to generate @code{CMPXCHG16B} instructions.
-@code{CMPXCHG16B} allows for atomic operations on 128-bit double quadword
-(or oword) data types.
-This is useful for high-resolution counters that can be updated
-by multiple processors (or cores).  This instruction is generated as part of
-atomic built-in functions: see @ref{__sync Builtins} or
-@ref{__atomic Builtins} for details.
+This option enables GCC to generate @code{CMPXCHG16B} instructions in 64-bit
+code to implement compare-and-exchange operations on 16-byte aligned 128-bit
+objects.  This is useful for atomic updates of data structures exceeding one
+machine word in size.  The compiler uses this instruction to implement
+@ref{__sync Builtins}.  However, for @ref{__atomic Builtins} operating on
+128-bit integers, a library call is always used.

 @item -msahf
 @opindex msahf