Fixes #42933.

isConnected always returns false on an {{domxref("Attr")}} node, even when the attribute's ownerElement is connected to the document. This trips people up, so this adds a note explaining why.

As @caugner noted on the issue, this is consistent across Chrome, Firefox, and Safari, and is out of scope for BCD — the spec defines isConnected for all nodes, and since an attribute node has no parent, it has no root that is a document, so it is never "connected".

I confirmed the behavior in Chrome:

const el = document.createElement("div");
el.setAttribute("data-x", "1");
const attr = el.getAttributeNode("data-x");
document.body.appendChild(el);

el.isConnected;                 // true
attr.ownerElement.isConnected;  // true
attr.isConnected;               // false
attr.parentNode;                // null
attr.getRootNode() === attr;    // true (its root is itself, not the document)