atomic: merge (delete & copy) changes from trunk


svn path=/branches/release/boost/atomic/; revision=84456
Merge libraries.htm update.

svn path=/branches/release/libs/atomic/; revision=84504
Merge [84506] from trunk: Document that constructors initialize enable_shared_from_this. Fixes #8573.

svn path=/branches/release/boost/atomic/; revision=84507
merge [83504] from trunk

svn path=/branches/release/boost/atomic.hpp; revision=84525