Editorial: add note about when ToASCII equals ASCII lowercase

Many implementations currently skip ToASCII if domain is ASCII-only, but as discovered in #267 and #309 (comment), this can result in some undesirable behavior. Adding a note prevents implementers from making the mistake of thinking ToASCII is a no-op if the input is ASCII, and also provides a recommendation on how to properly optimize the ToASCII step.

url.bs

@@ -528,11 +528,17 @@ decisions made, i.e. whether to use the <a>same site</a> or <a>schemelessly same
 <var>domain</var> and an optional boolean <var>beStrict</var> (default false), runs these steps:
 
 <ol>
- <li><p>Let <var>result</var> be the result of running <a abstract-op lt=ToASCII>Unicode ToASCII</a>
- with <i>domain_name</i> set to <var>domain</var>, <i>UseSTD3ASCIIRules</i> set to
- <var>beStrict</var>, <i>CheckHyphens</i> set to false, <i>CheckBidi</i> set to true,
- <i>CheckJoiners</i> set to true, <i>Transitional_Processing</i> set to false,
- and <i>VerifyDnsLength</i> set to <var>beStrict</var>.
+ <li>
+  <p>Let <var>result</var> be the result of running <a abstract-op lt=ToASCII>Unicode ToASCII</a>
+  with <i>domain_name</i> set to <var>domain</var>, <i>UseSTD3ASCIIRules</i> set to
+  <var>beStrict</var>, <i>CheckHyphens</i> set to false, <i>CheckBidi</i> set to true,
+  <i>CheckJoiners</i> set to true, <i>Transitional_Processing</i> set to false,
+  and <i>VerifyDnsLength</i> set to <var>beStrict</var>.
+
+  <p class=note>If <var>beStrict</var> is false, <var>domain</var> is an <a>ASCII string</a>, and
+  <a>strictly splitting</a> <var>domain</var> on U+002E (.) does not produce any
+  <a for=list>item</a> that <a for=string>starts with</a> "<code>xn--</code>", this step is
+  equivalent to <a>ASCII lowercasing</a> <var>domain</var>.
 
  <li><p>If <var>result</var> is a failure value, <a>validation error</a>, return failure.