Rollup merge of #46387 - chrisduerr:master, r=QuietMisdreavus

Fix rustdoc item summaries that are headers

Rustoc item summaries that are headers were not displayed at all because
they started with whitespace.

This PR fixes this and now removes the whitespace and then displays the
block.

I'm not sure if the rustdoc test is written correctly, if there's anything to improve, just let me know. :)

This fixes #46377.

This is how it looks when rendered out now:
![Rendered](https://i.imgur.com/7u8jUAM.png)

src/librustdoc/html/render.rs

@@ -1819,7 +1819,9 @@ fn full_path(cx: &Context, item: &clean::Item) -> String {
 
 fn shorter<'a>(s: Option<&'a str>) -> String {
     match s {
-        Some(s) => s.lines().take_while(|line|{
+        Some(s) => s.lines()
+            .skip_while(|s| s.chars().all(|c| c.is_whitespace()))
+            .take_while(|line|{
             (*line).chars().any(|chr|{
                 !chr.is_whitespace()
             })