doc: buffer.fill() can zero-fill on invalid input

Note that buffer.fill() can zero-fill on some input types if no
valid fill data remains, but does nothing on other input types.

PR-URL: https://github.com/nodejs-private/node-private/pull/120
Fixes: https://github.com/nodejs-private/security/issues/193
Refs: https://github.com/nodejs-private/node-private/pull/118
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/api/buffer.md

@@ -1177,8 +1177,12 @@ console.log(Buffer.allocUnsafe(3).fill('\u0222'));
 // Prints: <Buffer c8 a2 c8>
 ```
 
-If `value` contains invalid characters, it is truncated; if no valid
-fill data remains, no filling is performed:
+If `value` contains invalid characters, it is truncated.
+
+If no valid fill data remains, then the buffer is either zero-filled or no
+filling is performed, depending on the input type. That behavior is dictated by
+compatibility reasons and was changed to throwing an exception in Node.js v10,
+so it's not recommended to rely on that.
 
 ```js
 const buf = Buffer.allocUnsafe(5);