*_tokenizer: add module doccomment

src/request_tokenizer.rs

@@ -1,3 +1,5 @@
+//! Module containing a tokenizer named [`RequestTokenizer`] for MPD requests.
+
 use crate::commands::*;
 
 #[derive(Clone, Debug)]

src/response_tokenizer.rs

@@ -1,3 +1,6 @@
+//! Module containing a tokenizer named [`ResponseAttributes`] for MPD responses,
+//! as well as some helper macros that makes it easier to create response parsers
+
 pub type GenericResponseResult<'a> = Result<GenericResponse<'a>, &'a str>;
 
 pub type GenericResponse<'a> = HashMap<&'a str, GenericResponseValue<'a>>;