Delegate whether to print docblocks to 'document'

Add test to check this resolves rust-lang#24838 and rust-lang#26871.

src/librustdoc/html/render.rs

@@ -2540,11 +2540,10 @@ fn render_impl(w: &mut fmt::Formatter, cx: &Context, i: &Impl, link: AssocItemLi
             _ => panic!("can't make docs for trait item with name {:?}", item.name)
         }
 
-        match link {
-            AssocItemLink::Anchor if !is_static || render_static => {
-                document(w, cx, item)
-            },
-            _ => Ok(()),
+        if !is_static || render_static {
+            document(w, cx, item)
+        } else {
+            Ok(())
         }
     }
 

src/test/rustdoc/manual_impl.rs

@@ -0,0 +1,26 @@
+// 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.
+
+pub trait T {
+    fn a_method(&self) -> usize;
+}
+
+// @has manual_impl/struct.S.html '//*[@class="trait"]' 'T'
+// @has - '//*[@class="docblock"]' 'Docs associated with the trait implementation.'
+// @has - '//*[@class="docblock"]' 'Docs associated with the trait method implementation.'
+pub struct S(usize);
+
+/// Docs associated with the trait implementation.
+impl T for S {
+    /// Docs associated with the trait method implementation.
+    fn a_method(&self) -> usize {
+        self.0
+    }
+}