Documentation: isEquivalentHTML: Fix incorrect spec

As of #22506 (May 2020), `isEquivalentHTML` short-circuits if its two
given strings are identical, and thus no longer treats invalid HTML
differently. Remove the mention of special handling, which had been
introduced in #8304 (August 2018).

packages/blocks/src/api/validation/index.js

@@ -590,8 +590,7 @@ export function isClosedByToken( currentToken, nextToken ) {
 
 /**
  * Returns true if the given HTML strings are effectively equivalent, or
- * false otherwise. Invalid HTML is not considered equivalent, even if the
- * strings directly match.
+ * false otherwise.
  *
  * @param {string} actual   Actual HTML string.
  * @param {string} expected Expected HTML string.