docs(quinn): Copy edit poll_read(_buf?) docs

Looks to the following for writing inspiration:

- https://docs.rs/tokio/latest/tokio/io/trait.AsyncRead.html#tymethod.poll_read
- https://doc.rust-lang.org/stable/std/io/trait.Read.html#tymethod.read

Achieves:

- Reflowing to 100 columns.
- Style-consistency with code-escaping and initial-line period.
- Distinguishing first lines between the two methods' doc comments.
- Hyperlink to SendStream::finish
- General writing style.

quinn/src/recv_stream.rs

@@ -94,14 +94,17 @@ impl RecvStream {
         .await
     }
 
-    /// Attempts to read from the stream into buf.
+    /// Attempts to read from the stream into the provided buffer
     ///
-    /// On success, returns Poll::Ready(Ok(num_bytes_read)) and places data in
-    /// the buf. If no data was read, it implies that EOF has been reached.
+    /// On success, returns `Poll::Ready(Ok(num_bytes_read))` and places data into `buf`. If this
+    /// returns zero bytes read (and `buf` has a non-zero length), that indicates that the remote
+    /// side has [`finish`]ed the stream and the local side has already read all bytes.
     ///
-    /// If no data is available for reading, the method returns Poll::Pending
-    /// and arranges for the current task (via cx.waker()) to receive a notification
-    /// when the stream becomes readable or is closed.
+    /// If no data is available for reading, this returns `Poll::Pending` and arranges for the
+    /// current task (via `cx.waker()`) to be notified when the stream becomes readable or is
+    /// closed.
+    ///
+    /// [`finish`]: crate::SendStream::finish
     pub fn poll_read(
         &mut self,
         cx: &mut Context,
@@ -112,15 +115,18 @@ impl RecvStream {
         Poll::Ready(Ok(buf.filled().len()))
     }
 
-    /// Attempts to read from the stream into buf.
+    /// Attempts to read from the stream into the provided buffer, which may be uninitialized
     ///
-    /// On success, returns Poll::Ready(Ok(())) and places data in
-    /// the buf. If no data was read and the buffer had a non null capacity,
-    /// it implies that EOF has been reached.
+    /// On success, returns `Poll::Ready(Ok(()))` and places data into the unfilled portion of
+    /// `buf`. If this does not write any bytes to `buf` (and `buf.remaining()` is non-zero), that
+    /// indicates that the remote side has [`finish`]ed the stream and the local side has already
+    /// read all bytes.
     ///
-    /// If no data is available for reading, the method returns Poll::Pending
-    /// and arranges for the current task (via cx.waker()) to receive a notification
-    /// when the stream becomes readable or is closed.
+    /// If no data is available for reading, this returns `Poll::Pending` and arranges for the
+    /// current task (via `cx.waker()`) to be notified when the stream becomes readable or is
+    /// closed.
+    ///
+    /// [`finish`]: crate::SendStream::finish
     pub fn poll_read_buf(
         &mut self,
         cx: &mut Context,