[LangRef] Clarify `dereferenceable` -> `nonnull` implication

[jdoerfert]

Summary

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

Came up in D17993.

[aqjune]

As discussed in D17993, it looks fine.

llvm/docs/LangRef.rst

``dereferenceable(<n>)``
     This indicates that the parameter or return pointer is dereferenceable. This
     attribute may only be applied to pointer typed parameters. A pointer that
     is dereferenceable can be loaded from speculatively without a risk of
     trapping. The number of bytes known to be dereferenceable must be provided
     in parentheses. It is legal for the number of bytes to be less than the
     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
     non-null and non-dereferenceable (up to ``<n>`` bytes) at the same
     time. All non-null pointers tagged with
     ``dereferenceable_or_null(<n>)`` are ``dereferenceable(<n>)``.
     For address space 0 ``dereferenceable_or_null(<n>)`` implies that
     a pointer is exactly one of ``dereferenceable(<n>)`` or ``null``,