hl: fix incorrect hvl_t comment in H5IM__class_attr_equals

VL strings use char* in memory, not hvl_t (which is only for VL
sequences). Correct the comment per jhendersonHDF review on PR HDFGroup#6371.

Author: brtnfld

hl/src/H5IM.c

@@ -1010,7 +1010,8 @@ H5IM__class_attr_equals(hid_t loc_id, const char *dset_name, const char *expecte
     if (H5T_STRING != H5Tget_class(atid))
         goto out;
 
-    /* Reject VLEN strings; H5Aread into a char* buffer would write an hvl_t.
+    /* Reject VLEN strings; H5Aread into a char* buffer would overwrite it
+     * with a heap-allocated char* pointer rather than the string content.
      * Return 0 (not this class) rather than -1 (error): the attribute is
      * well-formed, it just cannot be the expected class string. */
     if ((isvl = H5Tis_variable_str(atid)) < 0)