Improve the explanation about the behaviour of read_line

library/std/src/io/mod.rs

@@ -2110,7 +2110,7 @@ fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
     }
 
     /// Read all bytes until a newline (the `0xA` byte) is reached, and append
-    /// them to the provided buffer.
+    /// them to the provided buffer (without clear buffer before appending to it).
     ///
     /// This function will read bytes from the underlying stream until the
     /// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes