[docs] Update __atomic builtins documentation.

[docs] Update __atomic builtins documentation.

	* doc/extend.texi (__atomic Builtins): Move implementation details
	to the end of the description, rewrite opening paragraphs, state
	difference with __sync builtins, state C11/C++11 assumptions,
	weaken itemized descriptions, add explanation of memory model
	behaviour, expand description of compare-exchange, simplify text.

From-SVN: r222803