doc: StringDecoder supported encodings

[thany]

See here:
https://nodejs.org/docs/latest-v12.x/api/string_decoder.html

It should specify which encodings are supported by StringDecoder. The ones I've found are utf8 which is documented, and ucs2 which is not mentioned, so I found it purely by chance.

We have no way of knowing what other encodings are supported.

[himself65]

encoding parameter both pass through these functions

node/lib/internal/util.js

Lines 122 to 176 in 21bd667

	// Return undefined if there is no match.
	// Move the "slow cases" to a separate function to make sure this function gets
	// inlined properly. That prioritizes the common case.
	function normalizeEncoding(enc) {
	if (enc == null || enc === 'utf8' || enc === 'utf-8') return 'utf8';
	return slowCases(enc);
	}
	function slowCases(enc) {
	switch (enc.length) {
	case 4:
	if (enc === 'UTF8') return 'utf8';
	if (enc === 'ucs2' || enc === 'UCS2') return 'utf16le';
	enc = `${enc}`.toLowerCase();
	if (enc === 'utf8') return 'utf8';
	if (enc === 'ucs2') return 'utf16le';
	break;
	case 3:
	if (enc === 'hex' || enc === 'HEX' || `${enc}`.toLowerCase() === 'hex')
	return 'hex';
	break;
	case 5:
	if (enc === 'ascii') return 'ascii';
	if (enc === 'ucs-2') return 'utf16le';
	if (enc === 'UTF-8') return 'utf8';
	if (enc === 'ASCII') return 'ascii';
	if (enc === 'UCS-2') return 'utf16le';
	enc = `${enc}`.toLowerCase();
	if (enc === 'utf-8') return 'utf8';
	if (enc === 'ascii') return 'ascii';
	if (enc === 'ucs-2') return 'utf16le';
	break;
	case 6:
	if (enc === 'base64') return 'base64';
	if (enc === 'latin1' || enc === 'binary') return 'latin1';
	if (enc === 'BASE64') return 'base64';
	if (enc === 'LATIN1' || enc === 'BINARY') return 'latin1';
	enc = `${enc}`.toLowerCase();
	if (enc === 'base64') return 'base64';
	if (enc === 'latin1' || enc === 'binary') return 'latin1';
	break;
	case 7:
	if (enc === 'utf16le' || enc === 'UTF16LE' ||
	`${enc}`.toLowerCase() === 'utf16le')
	return 'utf16le';
	break;
	case 8:
	if (enc === 'utf-16le' || enc === 'UTF-16LE' ||
	`${enc}`.toLowerCase() === 'utf-16le')
	return 'utf16le';
	break;
	default:
	if (enc === '') return 'utf8';
	}
	}

and I have opened a PR that add a link on the doc

[DerekNonGeneric]

I think this can be closed, right?

/cc @himself65