Auto merge of #26513 - shunyata:master, r=alexcrichton

I'm currently reading the rust book and this variable name tripped me up.
Because it was called "input", I thought at first it might contain the line
read by read_line(). This new variable name will be more instructive to rust
beginners.

src/doc/trpl/error-handling.md

@@ -225,7 +225,7 @@ There's another way of doing this that's a bit nicer than `unwrap()`:
 
 ```rust,ignore
 let mut buffer = String::new();
-let input = io::stdin().read_line(&mut buffer)
+let num_bytes_read = io::stdin().read_line(&mut buffer)
                                 .ok()
                                 .expect("Failed to read line");
 ```