Update clang-tidy documentation.

[angelgarcia]

Summary

Improve modernize-use-auto documentation (https://llvm.org/bugs/show_bug.cgi?id=24962).
Add documentation for modernize-make-unique.

[klimek]

LG

docs/clang-tidy/checks/list.rst

.. toctree::
    misc-static-assert
    misc-swapped-arguments
    misc-undelegated-constructor
    misc-uniqueptr-reset-release
    misc-unused-alias-decls
    misc-unused-parameters
    misc-unused-raii
    modernize-loop-convert
+   modernize-make-unique
    modernize-pass-by-value
    modernize-replace-auto-ptr
    modernize-shrink-to-fit
    modernize-use-auto
    modernize-use-nullptr
    modernize-use-override
    readability-braces-around-statements
    readability-container-size-empty
    readability-else-after-return
    readability-function-size
    readability-identifier-naming
    readability-inconsistent-declaration-parameter-name
    readability-named-parameter
    readability-redundant-smartptr-get
    readability-redundant-string-cstr
    readability-simplify-boolean-expr

docs/clang-tidy/checks/modernize-make-unique.rst

+modernize-make-unique
+=====================
+
+This check finds the creation of ``std::unique_ptr`` objects by explicitly
+calling the constructor and a ``new`` expression, and replaces it with a call
+to ``std::make_unique``, introduced in C++14.
+
+.. code-block:: c++
+
+  auto my_ptr = std::unique_ptr<MyPair>(new MyPair(1, 2));
+
+  // becomes
+
+  auto my_ptr = std::make_unique<MyPair>(1, 2);
+

docs/clang-tidy/checks/modernize-use-auto.rst

.. code-block:: c++
   // The following implementation-specific iterator type for which
   // std::vector<int>::iterator could be a typedef would not be transformed.
   __gnu_cxx::__normal_iterator<int*, std::vector> K = MyVec.begin();
 
 * The initializer for the variable being declared is not a braced initializer
   list. Otherwise, use of ``auto`` would cause the type of the variable to be
   deduced as``std::initializer_list``.
 
+New expressions
+---------------
+
+Frequently, when a pointer is declared and initialized with ``new``, the
+pointee type has to be written twice: in the declaration type and in the
+``new`` expression. In this cases, the declaration type can be replaced with
+``auto`` improving readability and maintainability.
+
+.. code-block:: c++
+
+  TypeName *my_pointer = new TypeName(my_param);
+
+  // becomes
+
+  auto my_pointer = new TypeName(my_param);
+
+The check will also replace the declaration type in multiple declarations, if
+the following conditions are satisfied:
+
+* All declared variables have the same type (i.e. all of them are pointers to
+  the same type).
+* All declared variables are initialized with a ``new`` expression.
+* The types of all the new expressions are the same than the pointee of the
+  declaration type.
+
+.. code-block:: c++
+
+  TypeName *my_first_pointer = new TypeName, *my_second_pointer = new TypeName;
+
+  // becomes
+
+  auto my_first_pointer = new TypeName, my_second_pointer = new TypeName;
+
 Known Limitations
 -----------------
 * If the initializer is an explicit conversion constructor, the check will not
   replace the type specifier even though it would be safe to do so.
 
 * User-defined iterators are not handled at this time.