add #define PERL_NO_GET_CONTEXT to ParseXS's pods

Not using PERL_NO_GET_CONTEXT is for legacy or unmaintained pre-ithreads
code. New code should define it.
[perl #121223] explain PERL_NO_GET_CONTEXT a little

and tell the reader where to find the details.
[perl #121223] encourage use of PERL_NO_GET_CONTEXT