Document inability to match lone low surrogates accurately

[mathiasbynens]

var regex = /^a[\u{D800}-\u{DBFF}\u{DC00}-\u{DFFF}]b$/u;
console.log(
  regex.test('a\uD800b'),
  regex.test('a\uDC00b')
);

Expected: true, true
Actual: true, false

https://mothereff.in/regexpu#var%20regex%20%3D%20%2F%5Ea%5B%5Cu%7BD800%7D-%5Cu%7BDBFF%7D%5Cu%7BDC00%7D-%5Cu%7BDFFF%7D%5Db%24%2Fu%3B%0Aconsole.log%28%0A%20%20regex.test%28%27a%5CuD800b%27%29%2C%0A%20%20regex.test%28%27a%5CuDC00b%27%29%0A%29%3B

Ref. mathiasbynens/regenerate#28 (comment).
https://esdiscuss.org/topic/q-lonely-surrogates-and-unicode-regexps#content-3