[Syntax Breaking] Rename DefaultImpl to AutoImpl

DefaultImpl is a highly confusing name for what we now call auto impls,
as in `impl Send for ..`. The name auto impl is not formally decided
but for sanity anything is better than `DefaultImpl` which refers
neither to `default impl` nor to `impl Default`.

src/librustdoc/passes/mod.rs

@@ -116,7 +116,7 @@ impl<'a> fold::DocFolder for Stripper<'a> {
             // handled in the `strip-priv-imports` pass
             clean::ExternCrateItem(..) | clean::ImportItem(..) => {}
 
-            clean::DefaultImplItem(..) | clean::ImplItem(..) => {}
+            clean::AutoImplItem(..) | clean::ImplItem(..) => {}
 
             // tymethods/macros have no control over privacy
             clean::MacroItem(..) | clean::TyMethodItem(..) => {}