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

docs/ReleaseNotes.rst

 
 - ``-Wtautological-compare`` now warns when comparing an unsigned integer and 0
   regardless of whether the constant is signed or unsigned."
 
 - ``-Wtautological-compare`` now warns about comparing a signed integer and 0
   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
 -------------------------------------------------
 
 - Bitrig OS was merged back into OpenBSD, so Bitrig support has been
   removed from Clang/LLVM.
 
 New Compiler Flags
 ------------------