docs(arg_tokenizer): restore flag_name_matches's own doc comment

An earlier edit inserted is_digit_run's doc block directly above
flag_name_matches's without a blank line and without flag_name_matches's
own doc reattached to its signature, so cargo doc rendered
flag_name_matches's explanation on is_digit_run and left
flag_name_matches itself undocumented. Reordered so each function keeps
its own doc block.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>