Update an attribute's node document

When an attribute is appended to an element or is replacing an existing attribute, we need to ensure that its node document remains accurate.

Fixes #1359.

dom.bs

@@ -6553,6 +6553,9 @@ steps:
 
  <li><p>Set <var>attribute</var>'s <a for=Attr>element</a> to <var>element</var>.
 
+ <li><p>Set <var>attribute</var>'s <a for=Node>node document</a> to <var>element</var>'s
+ <a for=Node>node document</a>.
+
  <li><p><a>Handle attribute changes</a> for <var>attribute</var> with <var>element</var>, null, and
  <var>attribute</var>'s <a for=Attr>value</a>.
 </ol>
@@ -6582,6 +6585,9 @@ steps:
  <li><p>Set <var>newAttr</var>'s <a for=Attr>element</a> to <var>oldAttr</var>'s
  <a for=Attr>element</a>.
 
+ <li><p>Set <var>newAttr</var>'s <a for=Node>node document</a> to <var>oldAttr</var>'s
+ <a for=Attr>element</a>'s <a for=Node>node document</a>.
+
  <li><p>Set <var>oldAttr</var>'s <a for=Attr>element</a> to null.
 
  <li><p><a>Handle attribute changes</a> for <var>oldAttr</var> with <var>newAttr</var>'s
@@ -7473,15 +7479,15 @@ to as <em>content attributes</em> to avoid confusion with IDL attributes.
 
 <p class=note>User agents could have this as an internal slot as an optimization.
 
-When an <a>attribute</a> is created, its
+<p>When an <a>attribute</a> is created, its
 <a for=Attr>local name</a> is given. Unless explicitly
 given when an <a>attribute</a> is created, its
 <a for=Attr>namespace</a>,
 <a for=Attr>namespace prefix</a>, and
 <a for=Attr>element</a> are set to null, and its
 <a for=Attr>value</a> is set to the empty string.
 
-An
+<p>An
 <dfn export id=concept-named-attribute lt="named attribute"><code><var>A</var></code> attribute</dfn>
 is an <a>attribute</a> whose
 <a for=Attr>local name</a> is