[clang-tidy] Add an example for misc-unused-alias-decls

Differential Revision: https://reviews.llvm.org/D89270

clang-tools-extra/docs/clang-tidy/checks/misc-unused-alias-decls.rst

@@ -5,3 +5,10 @@ misc-unused-alias-decls
 
 
 Finds unused namespace alias declarations.
+
+.. code-block:: c++
+
+  namespace my_namespace {
+  class C {};
+  }
+  namespace unused_alias = ::my_namespace;