Editorial: Structured header for PadISOYear

While looking for "is an integer" assertions (#1467), I decided to remove
this one by converting the AO to have a structured header.

Author: ptomato

spec/plaindate.html

@@ -969,10 +969,17 @@ <h1>BalanceISODate (
       </emu-alg>
     </emu-clause>
 
-    <emu-clause id="sec-temporal-padisoyear" aoid="PadISOYear">
-      <h1>PadISOYear ( _y_ )</h1>
+    <emu-clause id="sec-temporal-padisoyear" type="abstract operation">
+      <h1>
+        PadISOYear (
+          _y_: an integer,
+        ): a String
+      </h1>
+      <dl class="header">
+        <dt>description</dt>
+        <dd>It returns a String representation of _y_ suitable for inclusion in an ISO 8601 string, either in 4-digit format or 6-digit format with sign.</dd>
+      </dl>
       <emu-alg>
-        1. Assert: _y_ is an integer.
         1. If _y_ &ge; 0 and _y_ &le; 9999, then
           1. Return ToZeroPaddedDecimalString(_y_, 4).
         1. If _y_ &gt; 0, let _yearSign_ be *"+"*; otherwise, let _yearSign_ be *"-"*.