doc: remove "if provided" for optional arguments

Remove "if provided" when discussing arguments that are explicitly
indicated to be optional and have default values.

PR-URL: #19690
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott

doc/api/buffer.md

@@ -461,8 +461,8 @@ changes:
 * `string` {string} String to encode.
 * `encoding` {string} The encoding of `string`. **Default:** `'utf8'`
 
-Creates a new `Buffer` containing `string`. If provided, the `encoding`
-parameter identifies the character encoding of `string`.
+Creates a new `Buffer` containing `string`. The `encoding` parameter identifies
+the character encoding of `string`.
 
 ```js
 const buf1 = new Buffer('this is a tést');
@@ -839,8 +839,8 @@ added: v5.10.0
 * `string` {string} A string to encode.
 * `encoding` {string} The encoding of `string`. **Default:** `'utf8'`
 
-Creates a new `Buffer` containing `string`. If provided, the `encoding`
-parameter identifies the character encoding of `string`.
+Creates a new `Buffer` containing `string`. The `encoding` parameter identifies
+the character encoding of `string`.
 
 ```js
 const buf1 = Buffer.from('this is a tést');