List raw string literals for # in index

For `#`, we had noted that it was used in attributes, but it's also
used in raw string literals, raw byte string literals, and raw C
string literals, so let's list those as well.

Author: traviscross

src/syntax-index.md

@@ -110,7 +110,7 @@ This appendix provides an index of tokens and common forms with links to where t
 | `->`   | RArrow      | [functions], [closures], [function pointer type] |
 | `=>`   | FatArrow    | [match arms][match], [macros] |
 | `<-`   | LArrow      | The left arrow symbol has been unused since before Rust 1.0, but it is still treated as a single token. |
-| `#`    | Pound       | [attributes] |
+| `#`    | Pound       | [attributes], [raw string literals], [raw byte string literals], [raw C string literals] |
 | `$`    | Dollar      | [macros] |
 | `?`    | Question    | [try propagation expressions][question], [questionably sized][sized], [macro Kleene matcher] |
 | `~`    | Tilde       | The tilde operator has been unused since before Rust 1.0, but its token may still be used. |