diagnostics: if AssocFn has self argument, describe as method

Discussed in
https://rust-lang.zulipchat.com/#narrow/stream/147480-t-compiler.2Fwg-diagnostics/topic/.22associated.20function.22.20vs.20.22method.22/near/329265515

This commit also changes the tooltips on rustdoc intra-doc links
targeting methods.

compiler/rustc_hir_analysis/src/coherence/orphan.rs

@@ -531,7 +531,7 @@ fn lint_auto_trait_impl<'tcx>(
         }),
         |lint| {
             let item_span = tcx.def_span(self_type_did);
-            let self_descr = tcx.def_kind(self_type_did).descr(self_type_did);
+            let self_descr = tcx.def_descr(self_type_did);
             match arg {
                 ty::util::NotUniqueParam::DuplicateParam(arg) => {
                     lint.note(&format!("`{}` is mentioned multiple times", arg));