[selectors-4] Clarify :blank

[fantasai]

See whatwg/html#8451 (comment)

Rule of thumb: “if it always submits, and would do so with an empty string, it's :blank; if it sometimes submits, and is set to not submit, it's :blank”. Action buttons (rather than toggle buttons that represent a state) such as <input type=reset>, <button>, <input type=submit>, etc., never match :blank.

[tabatkins]

Changed the :blank example into a note that reproduces the guidance from the HTML issue (which we expect HTML to adopt).