[LangRef] Clarify dereferenceable -> nonnull implication

If `null_pointer_is_valid` is present, `dereferenceable` does not imply
`nonnull`, make it clear.

Came up in D17993.

Reviewed By: aqjune

Differential Revision: https://reviews.llvm.org/D89417

llvm/docs/LangRef.rst

@@ -1234,7 +1234,8 @@ Currently, only the following parameter attributes are defined:
     size of the pointee type. The ``nonnull`` attribute does not imply
     dereferenceability (consider a pointer to one element past the end of an
     array), however ``dereferenceable(<n>)`` does imply ``nonnull`` in
-    ``addrspace(0)`` (which is the default address space).
+    ``addrspace(0)`` (which is the default address space), except if the
+    ``null_pointer_is_valid`` function attribute is present.
 
 ``dereferenceable_or_null(<n>)``
     This indicates that the parameter or return value isn't both