Rollup merge of #88742 - GuillaumeGomez:fix-table-in-docblocks, r=nbdd0121

Fix table in docblocks

"Overwrite" of #88702.

Instead of adding a z-index to the sidebar (which only hides the issue, doesn't fix it), I wrap `<table>` elements inside a `<div>` and limit all chidren of `.docblock` elements' width to prevent having the scrollbar on the whole doc block.

![Screenshot from 2021-09-08 15-11-24](https://user-images.githubusercontent.com/3050060/132515740-71796515-e74f-429f-ba98-2596bdbf781c.png)

Thanks `@nbdd0121` for `overflow-x: auto;`. ;)

r? `@notriddle`

src/librustdoc/html/render/print_item.rs

@@ -275,7 +275,8 @@ fn item_module(w: &mut Buffer, cx: &Context<'_>, item: &clean::Item, items: &[cl
             write!(
                 w,
                 "<h2 id=\"{id}\" class=\"section-header\">\
-                       <a href=\"#{id}\">{name}</a></h2>\n{}",
+                    <a href=\"#{id}\">{name}</a>\
+                 </h2>\n{}",
                 ITEM_TABLE_OPEN,
                 id = cx.derive_id(short.to_owned()),
                 name = name