doc: readline 'line' event emits final line

Updated docs to reflect current behaviour of readline:
final line of input will be emitted via `'line'` event
when input stream `'end'` event is emitted even when
the input is not newline terminated.

Refs: nodejs/node-v0.x-archive#7238

PR-URL: #42214
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/readline.md

@@ -82,6 +82,9 @@ The `'line'` event is emitted whenever the `input` stream receives an
 end-of-line input (`\n`, `\r`, or `\r\n`). This usually occurs when the user
 presses <kbd>Enter</kbd> or <kbd>Return</kbd>.
 
+The `'line'` event is also emitted if new data has been read from a stream and
+that stream ends without a final end-of-line marker.
+
 The listener function is called with a string containing the single line of
 received input.