better POD for mg_get and SvGROW

better POD for mg_get and SvGROW

SvGROW unconditionally derefs SvANY to check SvLEN. A crash occurs if the
sv is SVt_NULL. Also mg_get uses SvMAGIC which also has the same problem.
Rather than having people finding these properties out by trial and error,
document them. There is no sense in adding type checks since these 2 calls
have been had sv type restrictions since probably day 1 and for
performance reason. Anyone who hit the restrictions would have either
fixed their code immediately, or abandoned using XS. I observed someone
abandoning XS in the field over these undocumented restrictions.