doc: fix an output example in repl.md

Make `_` reassignment example match more with the current output.
Extend the example for more clarity.

PR-URL: https://github.com/nodejs/node/pull/10244
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/repl.md

@@ -140,6 +140,7 @@ global or scoped variable, the input `fs` will be evaluated on-demand as
 
 The default evaluator will, by default, assign the result of the most recently
 evaluated expression to the special variable `_` (underscore).
+Explicitly setting `_` to a value will disable this behavior.
 
 ```js
 > [ 'a', 'b', 'c' ]
@@ -147,11 +148,14 @@ evaluated expression to the special variable `_` (underscore).
 > _.length
 3
 > _ += 1
+Expression assignment to _ now disabled.
+4
+> 1 + 1
+2
+> _
 4
 ```
 
-Explicitly setting `_` to a value will disable this behavior.
-
 ### Custom Evaluation Functions
 
 When a new `repl.REPLServer` is created, a custom evaluation function may be