Document return value of clock handler function

[kemstevens]

I noticed that my clock handlers were only being called once unless my handler function returns false. I "get it", but it seems like this interesting behavior related to the return value should be documented. I checked the doxygen docs for SST::Clock::HandlerBase and it says nothing about what the return type means. (http://sst-simulator.org/SSTDoxygen/8.0.0_docs/html/classSST_1_1Clock_1_1HandlerBase.html#details)

May I suggest a small tweak to the code like this: in sstcore-8.0.0/src/sst/core/clock.h at line 46:

OLD: /** Function called when Handler is invoked. */

NEW: /** Function called when Handler is invoked. @return true if this clock's work is complete or return false if the core should continue calling the handler in the future. */