Added escapes to docs. Fixes
//! | `'a'..'z'` | matches one character between `'a'` and `'z'` |
//! | `a` | matches rule `a` |
//!
+//! Strings and characters follow
+//! [Rust's escape mechanisms](https://doc.rust-lang.org/reference/tokens.html#byte-escapes), while
+//! identifiers can contain alpha-numeric characters and underscores (`_`), as long as they do not
+//! start with a digit.
+//!
//! 2. Non-terminals
//!
//! | Non-terminal | Usage |
