try to clarify PERL_NO_GET_CONTEXT (again)

- it speeds up XS code under a threaded perl, does not affect
  non-threaded perls
- note that it does require (minimal) extra work when defining
  helper C functions
- try to clarify when the extra fetches happen

Best viewed with --word-diff
try to clarify why hv_store_ent causes the modification of read-only value error

the current phrasing seems to attribute to hv_store(_ent) the same behaviour as
Perl-space assignment. Expand it a bit so the difference is (hopefully)
clearer.