better POD for SvPVX, pre-5.9.3 Perls

PV/char * was moved to SV head in commit 7b2c381cf3 . Prior to this, PV
was in SV body, and sv_any can be NULL, so accessing SvPVX without
checking type on old Perls was a SEGV. Make a note of this so others
don't find out the hard way.