rustdoc: Handle tests with bare # marks

Strip them from output like other `# `-starting lines.

Closes #23106

src/librustdoc/html/markdown.rs

@@ -183,7 +183,9 @@ fn as_bytes(&self) -> &[u8] {
 /// left as-is.)
 fn stripped_filtered_line<'a>(s: &'a str) -> Option<&'a str> {
     let trimmed = s.trim();
-    if trimmed.starts_with("# ") {
+    if trimmed == "#" {
+        Some("")
+    } else if trimmed.starts_with("# ") {
         Some(&trimmed[2..])
     } else {
         None

src/test/rustdoc/issue-23106.rs

@@ -0,0 +1,17 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags:--test
+
+/// ```
+/// #
+/// ```
+pub fn main() {
+}