[boost] [thread] why is 'thread_group' in 'boost/thread/detail'?

as of 1.53 the docs say the 'thread_group' class is included by this instruction: #include <boost/thread/thread_group.hpp> http://www.boost.org/doc/libs/1_53_0/doc/html/thread/thread_management.html#... whereas the actual file is not in the 'boost/thread', but accidentally is in 'boost/thread/detail' is that intentional? -- Pavel P.S. if you notice a grammar mistake or weird phrasing in my message please point it out