Add a hint what BufRead functions do on EOF

Author: tbu-

src/libstd/io/mod.rs

@@ -1410,6 +1410,8 @@ pub trait BufRead: Read {
     ///
     /// If successful, this function will return the total number of bytes read.
     ///
+    /// An empty buffer returned indicates that the stream has reached EOF.
+    ///
     /// # Errors
     ///
     /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
@@ -1470,6 +1472,8 @@ pub trait BufRead: Read {
     ///
     /// If successful, this function will return the total number of bytes read.
     ///
+    /// An empty buffer returned indicates that the stream has reached EOF.
+    ///
     /// # Errors
     ///
     /// This function has the same error semantics as [`read_until`] and will