Fix doc about fiber::operator safe_bool(): NOT same as joinable().
Looking at the implementation, it seems that operator safe_bool() and its
companion, operator!(), interrogate whether the referenced fiber has or has
not terminated. joinable() does not.
Add more joinable() cross-references, and cross-reference joinable(), operator
safe_bool() and operator!().
Merge pull request #7 from nat-goodspeed/fiber-mt

Fix doc about fiber::operator safe_bool(): NOT same as joinable().