pod: mention SvPVCLEAR()

pod/perlguts.pod

@@ -186,7 +186,7 @@ sv_insert() or sv_insert_flags().
 If you don't need the existing content of the SV, you can avoid some
 copying with:
 
-    sv_setpvs(sv, "");
+    SvPVCLEAR(sv);
     s = SvGROW(sv, needlen + 1);
     /* something that modifies up to needlen bytes at s, but modifies
        newlen bytes

pod/perlhacktut.pod

@@ -74,7 +74,7 @@ of C<pat>:
 
     items = SP - MARK;
     MARK++;
-    sv_setpvs(cat, "");
+    SvPVCLEAR(cat);
  +  patcopy = pat;
     while (pat < patend) {