src/doc/complement-lang-faq.md mentions str indexing, which was removed

[SimonSapin]

Scanning a str for ASCII-range codepoints can still be done safely octet-at-a-time, with each indexing operation pulling out a u8 costing only O(1) and producing a value that can be cast and compared to an ASCII-range char.

Indexing on &str used to return u8 but is now an error: strings do not support indexing at all.

This paragraph will need to be rephrased. Maybe suggest using .as_bytes(), where the index in the resulting &[u8] of an ASCII-range byte can safely be used e.g. to slice the original str.

CC @steveklabnik