message: Clarify comment in get_strconv_attribute
  This might have helped a bit with confusion in
diff: @@ -2878,7 +2878,7 @@ PUGI__NS_BEGIN
  {
  PUGI__STATIC_ASSERT(parse_escapes == 0x10 && parse_eol == 0x20 && parse_wconv_attribute == 0x40 && parse_wnorm_attribute == 0x80);
  - switch ((optmask >> 4) & 15) // get bitmask for flags (wconv wnorm eol escapes)
  + switch ((optmask >> 4) & 15) // get bitmask for flags (wnorm wconv eol escapes); this simultaneously checks 4 options from assertion above
  {
  case 0: return strconv_attribute_impl<opt_false>::parse_simple;
  case 1: return strconv_attribute_impl<opt_true>::parse_simple;
  