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

[hfinkel]

ReleaseNotes.rst

82

We can probably just make this one entry:

``-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).

[sylvestre.ledru]

Thanks, it makes sense!
Landed in r314387

ReleaseNotes.rst

   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. It has an undefined behavior if the offset is nonzero.
+
+- ``-Wnull-pointer-arithmetic`` 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
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