stipulate sep parser must consume (#1723)

the separator parser cannot be any parser but must be a consuming parser, contrary to documentation. Ideally, we should have a trait to assert this constraint as well.

src/multi/mod.rs

@@ -337,7 +337,7 @@ where
 /// [`cut`][crate::combinator::cut].
 ///
 /// # Arguments
-/// * `sep` Parses the separator between list elements.
+/// * `sep` Parses the separator between list elements. Must be consuming. 
 /// * `f` Parses the elements of the list.
 ///
 /// ```rust
@@ -458,7 +458,7 @@ where
 /// [`cut`][crate::combinator::cut].
 ///
 /// # Arguments
-/// * `sep` Parses the separator between list elements.
+/// * `sep` Parses the separator between list elements. Must be consuming.
 /// * `f` Parses the elements of the list.
 /// ```rust
 /// # use nom::{Err, error::{Error, ErrorKind}, Needed, IResult, Parser};