enhance the documentation of std::io::BufReader regarding potential data loss

Author: Marco Conte

src/libstd/io/buffered.rs

@@ -24,7 +24,8 @@ use crate::memchr;
 ///
 /// When the `BufReader<R>` is dropped, the contents of its buffer will be
 /// discarded. Creating multiple instances of a `BufReader<R>` on the same
-/// stream can cause data loss.
+/// stream can cause data loss, as well as reading from the underlying reader
+/// after unwrapping the `BufReader<R>` instance with `BufReader<R>::into_inner()`.
 ///
 /// [`Read`]: ../../std/io/trait.Read.html
 /// [`TcpStream::read`]: ../../std/net/struct.TcpStream.html#method.read
@@ -179,7 +180,8 @@ impl<R> BufReader<R> {
 
     /// Unwraps this `BufReader<R>`, returning the underlying reader.
     ///
-    /// Note that any leftover data in the internal buffer is lost.
+    /// Note that any leftover data in the internal buffer is lost. Therefore,
+    /// a following read from the underlying reader may lead to data loss.
     ///
     /// # Examples
     ///