[doc] Add links to cpprefenence in synchronization primitives

Author: Pawel Karczewski

include/libpmemobj++/condition_variable.hpp

@@ -26,8 +26,9 @@ namespace obj
  * Persistent memory resident condition variable.
  *
  * This class is an implementation of a PMEM-resident condition
- * variable which mimics in behavior the C++11 std::condition_variable. The
- * typical usage example would be:
+ * variable which mimics in behavior the C++11
+ * [std::condition_variable](https://en.cppreference.com/w/cpp/thread/condition_variable).
+ * The typical usage example would be:
  * @snippet mutex/mutex.cpp cond_var_example
  * @ingroup synchronization
  */

include/libpmemobj++/mutex.hpp

@@ -23,9 +23,10 @@ namespace obj
  * Persistent memory resident mutex implementation.
  *
  * This class is an implementation of a PMEM-resident mutex
- * which mimics in behavior the C++11 std::mutex. This class
- * satisfies all requirements of the Mutex and StandardLayoutType
- * concepts. The typical usage example would be:
+ * which mimics in behavior the C++11
+ * [std::mutex](https://en.cppreference.com/w/cpp/thread/mutex). This class
+ * satisfies all requirements of the Mutex and StandardLayoutType concepts. The
+ * typical usage example would be:
  * @snippet mutex/mutex.cpp unique_guard_example
  * @ingroup synchronization
  */

include/libpmemobj++/shared_mutex.hpp

@@ -22,9 +22,10 @@ namespace obj
  * Persistent memory resident shared_mutex implementation.
  *
  * This class is an implementation of a PMEM-resident shared_mutex
- * which mimics in behavior the C++11 std::mutex. This class
- * satisfies all requirements of the SharedMutex and StandardLayoutType
- * concepts. The typical usage would be:
+ * which mimics in behavior the C++11
+ * [std::shared_mutex](https://en.cppreference.com/w/cpp/thread/shared_mutex).
+ * This class satisfies all requirements of the SharedMutex and
+ * StandardLayoutType concepts. The typical usage would be:
  * @snippet mutex/mutex.cpp shared_mutex_example
  * @ingroup synchronization
  */

include/libpmemobj++/timed_mutex.hpp

@@ -24,9 +24,10 @@ namespace obj
  * Persistent memory resident timed_mutex implementation.
  *
  * This class is an implementation of a PMEM-resident timed_mutex
- * which mimics in behavior the C++11 std::timed_mutex. This class
- * satisfies all requirements of the TimedMutex and StandardLayoutType
- * concepts. The typical usage example would be:
+ * which mimics in behavior the C++11
+ * [std::timed_mutex](https://en.cppreference.com/w/cpp/thread/timed_mutex).
+ * This class satisfies all requirements of the TimedMutex and
+ * StandardLayoutType concepts. The typical usage example would be:
  * @snippet mutex/mutex.cpp timed_mutex_example
  * @ingroup synchronization
  */