[Sema] Update release notes with details of implicit scalar to vector conversions

[sdardis]

Summary

Add notes on this to the C language section, along with the C++ section.

[hans]

lgtm, please commit to the 5.0 branch

[sdardis]

Thanks,
Simon

docs/ReleaseNotes.rst

 ---------------
 
 Clang's support for building native Windows programs ...
 
 
 C Language Changes in Clang
 ---------------------------
 
-- ...
+- Added near complete support for implicit scalar to vector conversion, a GNU
+  C/C++ language extension. With this extension, the following code is
+  considered valid:
+
+.. code-block:: c
+
+    typedef unsigned v4i32 __attribute__((vector_size(16)));
+
+    v4i32 foo(v4i32 a) {
+      // Here 5 is implicitly casted to an unsigned value and replicated into a
+      // vector with as many elements as 'a'.
+      return a + 5;
+    }
+
+The implicit conversion of a scalar value to a vector value--in the context of
+a vector expression--occurs when:
+
+- The type of the vector is that of a ``__attribute__((vector_size(size)))``
+  vector, not an OpenCL ``__attribute__((ext_vector_type(size)))`` vector type.
+
+- The scalar value can be casted to that of the vector element's type without
+  the loss of precision based on the type of the scalar and the type of the
+  vector's elements.
+
+- For compile time constant values, the above rule is weakened to consider the
+  value of the scalar constant rather than the constant's type.
+
+- Floating point constants with precise integral representations are not
+  implicitly converted to integer values, this is for compatability with GCC.
+
+
+Currently the basic integer and floating point types with the following
+operators are supported: ``+``, ``/``, ``-``, ``*``, ``%``, ``>``, ``<``,
+``>=``, ``<=``, ``==``, ``!=``, ``&``, ``|``, ``^`` and the corresponding
+assignment operators where applicable.
 
 ...
 
 C11 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 
 ...
 
 C++ Language Changes in Clang
 -----------------------------
 
+- As mentioned in `C Language Changes in Clang`_, Clang's support for
+  implicit scalar to vector conversions also applies to C++. Additionally
+  the following operators are also supported: ``&&`` and ``||``.
+
 ...
 
 C++1z Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
 ...
 
 Objective-C Language Changes in Clang