Add the new -Wnull-pointer-arithmetic warnings to the release notes

Differential Revision: https://reviews.llvm.org/D38186



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314387 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -78,6 +78,11 @@ Improvements to Clang's diagnostics
   when the signed integer is coerced to an unsigned type for the comparison.
   ``-Wsign-compare`` was adjusted not to warn in this case.
 
+- ``-Wnull-pointer-arithmetic`` now warns about performing pointer arithmetic
+  on a null pointer. Such pointer arithmetic has an undefined behavior if the
+  offset is nonzero. It also now warns about arithmetic on a null pointer
+  treated as a cast from integer to pointer (GNU extension).
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------