Improve code comments for some ctx stuff

Improve code comments for some ctx stuff

* in pp_return(), some comments were out of date about how
  leave_adjust_stacks() is called ;
* add a comment to all the functions that pp_return() tail-calls to the
  effect that they can be tail-called;
* make it clearer when/why OPf_SPECIAL is set on OP_LEAVE;
* CXt_LOOP_PLAIN can be a while loop as well as a plain block.