[LangRef] Clarify that the pointer after an objet must be valid.

In some places, we rely on the assumption that the pointer after the
object must also be valid and not overflow, but it does not seem to be
spelled out clearly in LangRef, unless I missed a reference.

The GetElementPtr section mentions that the maximum object size is half
the pointer index type space, but then the pointer past the object may
wrap. Clarify that the pointer after the object must also be valid.

This should match Alive2's semantics: https://alive2.llvm.org/ce/z/Dk8QFL
(https://github.com/AliveToolkit/alive2/blob/master/tools/transform.cpp#L1288)

Author: fhahn

llvm/docs/LangRef.rst

@@ -11721,8 +11721,9 @@ As a corollary, the only pointer in bounds of the null pointer in the default
 address space is the null pointer itself.
 
 These rules are based on the assumption that no allocated object may cross
-the unsigned address space boundary, and no allocated object may be larger
-than half the pointer index type space.
+the unsigned address space boundary, the pointer after the object must be valid,
+and no allocated object may be larger than half the pointer index type space
+- 1.
 
 If ``inbounds`` is present on a ``getelementptr`` instruction, the ``nusw``
 attribute will be automatically set as well. For this reason, the ``nusw``