Cut off plain text descriptions after headers

Before:

    The Rust Standard LibraryThe Rust Standard Library is the foundation of portable Rust software, a set of minimal and battle-tested shared abstractions for the broader Rust ecosystem. It offers core types, like `Vec<T>` and `Option<T>`, library-defined operations on language primitives, standard macros, I/O and multithreading, among many other things.

After:

    The Rust Standard Library

src/librustdoc/html/markdown.rs

@@ -1124,6 +1124,7 @@ crate fn plain_text_summary(md: &str) -> String {
             Event::HardBreak | Event::SoftBreak => s.push(' '),
             Event::Start(Tag::CodeBlock(..)) => break,
             Event::End(Tag::Paragraph) => break,
+            Event::End(Tag::Heading(..)) => break,
             _ => (),
         }
     }

src/librustdoc/html/markdown/tests.rs

@@ -230,6 +230,7 @@ fn test_plain_text_summary() {
     t("code `let x = i32;` ...", "code `let x = i32;` ...");
     t("type `Type<'static>` ...", "type `Type<'static>` ...");
     t("# top header", "top header");
+    t("# top header\n\nfollowed by some text", "top header");
     t("## header", "header");
     t("first paragraph\n\nsecond paragraph", "first paragraph");
     t("```\nfn main() {}\n```", "");