[analyzer] Move docs of SmartPtr to correct subcategory

[RedDocMD]

Summary

The docs of alpha.cplusplus.SmartPtr was incorrectly placed under
alpha.deadcode. Moved it to under alpha.cplusplus

[NoQ]

Absolutely.

clang/docs/analyzer/checkers.rst

  };
 
  void f() {
    A a;
    A b = std::move(a); // note: 'a' became 'moved-from' here
    a.foo();            // warn: method call on a 'moved-from' object 'a'
  }
 
+.. _alpha-cplusplus-SmartPtr:
+
+alpha.cplusplus.SmartPtr (C++)
+""""""""""""""""""""""""""""""
+Check for dereference of null smart pointers.
+
+.. code-block:: cpp
+
+ void deref_smart_ptr() {
+   std::unique_ptr<int> P;
+   *P; // warn: dereference of a default constructed smart unique_ptr
+ }
+
+
 alpha.deadcode
 ^^^^^^^^^^^^^^
 .. _alpha-deadcode-UnreachableCode:
 
 alpha.deadcode.UnreachableCode (C, C++)
 """""""""""""""""""""""""""""""""""""""
 Check unreachable code.
 

  }
 
  // Objective-C
  void test(id x) {
    return;
    [x retain]; // warn
  }
 
-.. _alpha-cplusplus-SmartPtr:
-
-alpha.cplusplus.SmartPtr (C++)
-""""""""""""""""""""""""""""""
-Check for dereference of null smart pointers.
-
-.. code-block:: cpp
-
- void deref_smart_ptr() {
-   std::unique_ptr<int> P;
-   *P; // warn: dereference of a default constructed smart unique_ptr
- }
-
 alpha.fuchsia
 ^^^^^^^^^^^^^
 
 .. _alpha-fuchsia-lock:
 
 alpha.fuchsia.Lock
 """"""""""""""""""
 Similarly to :ref:`alpha.unix.PthreadLock <alpha-unix-PthreadLock>`, checks for