Explain why len and is_empty can be None in Body

src/body.rs

@@ -451,7 +451,7 @@
         })
     }
 
-    /// 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 @@
         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)
     }
@@ -578,7 +578,7 @@
 }
 
 impl AsyncRead for Body {
     #[allow(rustdoc::missing_doc_code_examples)]
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
@@ -602,7 +602,7 @@
 }
 
 impl AsyncBufRead for Body {
     #[allow(rustdoc::missing_doc_code_examples)]
     fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&'_ [u8]>> {
         self.project().reader.poll_fill_buf(cx)
     }