Rollup merge of rust-lang#94957 - iamzhangyong:explanation-read_line,…

… r=Dylan-DPC

Improve the explanation about the behaviour of read_line

Close issue like rust-lang/book#2574

library/std/src/io/mod.rs

@@ -2110,7 +2110,8 @@ pub trait BufRead: Read {
     }
 
     /// Read all bytes until a newline (the `0xA` byte) is reached, and append
-    /// them to the provided buffer.
+    /// them to the provided buffer. You do not need to clear the buffer before
+    /// appending.
     ///
     /// This function will read bytes from the underlying stream until the
     /// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes