buffer: add missing ARG_TYPE(ArrayBuffer) for isUtf8

[pluris]

Added due to inconsistencies between code and documentation.

node/doc/api/buffer.md

Lines 5200 to 5209 in 21211a3

	### `buffer.isUtf8(input)`
	<!-- YAML
	added:
	- v19.4.0
	- v18.14.0
	-->
	* input {Buffer | ArrayBuffer | TypedArray} The input to validate.
	* Returns: {boolean}

Reference was made to isAscii in the same API format.

node/doc/api/buffer.md

Lines 5184 to 5198 in 21211a3

	### `buffer.isAscii(input)`
	<!-- YAML
	added:
	- v19.6.0
	- v18.15.0
	-->
	* input {Buffer | ArrayBuffer | TypedArray} The input to validate.
	* Returns: {boolean}
	This function returns `true` if `input` contains only valid ASCII-encoded data,
	including the case in which `input` is empty.
	Throws if the `input` is a detached array buffer.

node/lib/buffer.js

Lines 1290 to 1296 in 21211a3

	function isAscii(input) {
	if (isTypedArray(input) || isAnyArrayBuffer(input)) {
	return bindingIsAscii(input);
	}
	throw new ERR_INVALID_ARG_TYPE('input', ['ArrayBuffer', 'Buffer', 'TypedArray'], input);
	}

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/58287/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/58324/

[nodejs-github-bot]

Landed in 3634f9c