clarify SvPV* and SvPV_force* docs

clarify SvPV* and SvPV_force* docs

See RT #116407.

Make it clear that SvPV(sv) does not always equal SvPVX(sv), and to use
SvPV_force() if necessary. Also, clarify that SvPV_force() will destroy
any non-string content like a reference. Then, make the other SvPV*
descriptions in general refer back to SvPV() or SvPV_force(), so that
people will spot these added caveats.