Explain why len and is_empty can be None

src/body.rs

@@ -451,7 +451,7 @@ impl Body {
         })
     }
 
-    /// Get the length of the body in bytes.
+    /// Get the length of the body in bytes if defined. [None] if not.
     ///
     /// # Examples
     ///
@@ -468,7 +468,7 @@ impl Body {
         self.length
     }
 
-    /// Returns `true` if the body has a length of zero, and `false` otherwise.
+    /// Returns `true` if the body has a length of zero, and `false` otherwise if length is defined. [None] if not.
     pub fn is_empty(&self) -> Option<bool> {
         self.length.map(|length| length == 0)
     }