Fix search results initial scroll position

When search results are rendered, the result cursor is set to the first
result and is scrolled into view via `scrollIntoView({block: "nearest"})`.
Typically, this call to `scrollIntoView` would be a no-op.  However,
6eb2e42 added a "TIP" at the top of the
results container.  Thus if the results container has `max-height: 0`,
the `scrollIntoView` call will set the scroll position below the "TIP".
Such is the case when clicking a `.query-button` or using the `?q=` URL
param, because the `.active` class is not yet applied to the results
container.

This commit modifies the CSS so that the "TIP" is only shown when the
`.active` class is applied.  Thus the `scrollIntoView` call will remain
a no-op even when the results container has `max-height: 0`.

lib/rdoc/generator/template/rails/resources/css/main.css

@@ -442,7 +442,9 @@ html {
   font-size: 0.9em;
   font-style: italic;
   color: color-mix(in srgb, currentColor 70%, transparent);
+}
 
+.panel__results.active::before {
   content: "TIP: Prefix query with \"#\" or \".\" to search for methods.";
 }