Documentation fix for std::num::strconv

- int_to_str_bytes_common() doesn't have a return value.
- float_to_str_bytes_common() has an old-style doc comment.
auto merge of #19354 : barosl/rust/strconv-doc-fix, r=steveklabnik

- `int_to_str_bytes_common()` doesn't have a return value.
- `float_to_str_bytes_common()` has an old-style doc comment.