Merge rust-bitcoin#3966: Change #[must_use] to be the same as stdlib

cb5ffde Change `#[must_use]` to be the same as stdlib (Jamil Lambert, PhD)

Pull request description:

  Change the iterator `#[must_use]` message to be the same as stdlib uses.

  Message taken from https://doc.rust-lang.org/src/core/iter/traits/iterator.rs.html#38

  Close rust-bitcoin#3962

ACKs for top commit:
  Kixunil:
    ACK cb5ffde
  tcharding:
    ACK cb5ffde

Tree-SHA512: 4866d9bddccdc28e2ba2b15cb4041851abb1512cea0317f20d2e2306df6dcd1addb828d61a84a2083d3ef47abb5cf9276e857cf098de2d28b4706ecd0a1f24fa

primitives/src/witness.rs

@@ -103,7 +103,7 @@ impl Witness {
     pub fn is_empty(&self) -> bool { self.witness_elements == 0 }
 
     /// Returns a struct implementing [`Iterator`].
-    #[must_use = "returned iterator should be used"]
+    #[must_use = "iterators are lazy and do nothing unless consumed"]
     pub fn iter(&self) -> Iter {
         Iter { inner: self.content.as_slice(), indices_start: self.indices_start, current_index: 0 }
     }