fix a doc comment

src/function.rs

@@ -804,8 +804,7 @@ impl Function {
  recursive: false,
  }
  }
- /// Get how many arguments this function pops off the stack and how many it pushes.
- /// Returns `None` if either of these values are dynamic.
+ /// Get the [`Signature`] of this function
  pub fn signature(&self) -> Signature {
  self.signature
  }