It respects SV_SMAGIC flag, not SV_GMAGIC (which it ignores).
Concatenates the string onto the end of the string which is in the SV. The
C<len> indicates number of bytes to copy. If the SV has the UTF-8
status set, then the bytes appended should be valid UTF-8.
-If C<flags> has C<SV_GMAGIC> bit set, will C<mg_get> on C<dsv> if
-appropriate, else not. C<sv_catpvn> and C<sv_catpvn_nomg> are implemented
+If C<flags> has the C<SV_SMAGIC> bit set, will
+C<mg_set> on C<dsv> afterwards if appropriate.
+C<sv_catpvn> and C<sv_catpvn_nomg> are implemented
in terms of this function.
=cut