major rewrite of pod:

• describe all possible forms of redirectors
  
• list available options
  
• explain how run3() works
  fix #31343 by using the three argument form of open() for files;
  unfortunately that isn't available before Perl 5.6.0,
  hence "use 5.00600"; also use lexical filehandles everywhere
  implement options append{stdout,stderr}; add test cases
  edit META.yml so that the profiling modules IPC::Run3::Prof*
  (which aren't of general use) won't show on CPAN