Clarify which ASCII values can appear unescaped in a clob

[zslayton]

Depending on where you look in the spec and/or ion-tests, you'll find conflicting information about which ASCII characters can appear unescaped in a clob. Some say "characters >= 0x20", but that excludes lots of whitespace characters that are < 0x20. Some say "displayable ASCII", but DEL (0x7F) is shown to be legal in one of the ion-tests.