This notes that the caller now has control of a reference count of the
returned SV.
Wording mostly suggested by Paul Evans
/*
=for apidoc av_pop
-Pops an SV off the end of the array. Returns C<&PL_sv_undef> if the array
-is empty.
+Removes one SV from the end of the array, reducing its size by one and
+returning the SV (transferring control of one reference count) to the
+caller. Returns C<&PL_sv_undef> if the array is empty.
Perl equivalent: C<pop(@myarray);>