Inhibit clicks on summary's children

A byproduct of using `<details>` and `<summary>` to show/hide detailed
documentation was that clicking any part of a method heading (or impl
heading) would show or hide the documentation. This was not super
noticeable because clicking a link inside the method heading would
navigate to that link. But clicking any unlinked black text in a method
heading would trigger the behavior.

That behavior was somewhat unexpected, and means that if you try to click
a type name in a method heading, but miss by a few pixels, you get a
confusing surprise.

This change inhibits that behavior by putting an event listener on most
summaries that cancels the event unless the event target was the summary
itself. In practice, that means it cancels the event unless the target
was the "[+]" / "[-]", because the rest of the heading is wrapped inside
a `<div>`, which is the target for anything that doesn't have a more
specific target.

src/librustdoc/html/static/js/main.js

@@ -886,6 +886,14 @@ function hideThemeButtonState() {
         }
     });
 
+    onEachLazy(document.querySelectorAll(".rustdoc-toggle > summary:not(.hideme)"), function(el) {
+        el.addEventListener("click", function(e) {
+            if (e.target.tagName != "SUMMARY") {
+                e.preventDefault();
+            }
+        })
+    });
+
     onEachLazy(document.getElementsByClassName("notable-traits"), function(e) {
         e.onclick = function() {
             this.getElementsByClassName('notable-traits-tooltiptext')[0]