Get-magic is only called on ssv. SV_SMAGIC is accepted, too.
Concatenates the string from SV C<ssv> onto the end of the string in
SV C<dsv>. Modifies C<dsv> but not C<ssv>. If C<flags> has C<SV_GMAGIC>
-bit set, will C<mg_get> on the SVs if appropriate, else not. C<sv_catsv>
+bit set, will C<mg_get> on the C<ssv>, if appropriate, before
+reading it. If the C<flags> contain C<SV_SMAGIC>, C<mg_set> will be
+called on the modified SV afterward, if appropriate. C<sv_catsv>
and C<sv_catsv_nomg> are implemented in terms of this function.
=cut */