[conv.lval] Improve a bit more the note about std::nullptr_t case

- "read the object" -> "access the object"
  The "read" was derived from the rule at [intro.execution], but it
  didn't fit well here.
- "the object" -> "the object to which the glvalue refers"
  Make it clear what "the object" refers to.

#2372 (review)

source/expressions.tex

@@ -631,7 +631,7 @@
 \item If \tcode{T} is \cv{}~\tcode{std::nullptr_t}, the result is a
 null pointer constant\iref{conv.ptr}.
 \begin{note}
-Since the conversion does not read the object,
+Since the conversion does not access the object to which the glvalue refers,
 there is no side effect even if \tcode{T} is volatile-qualified\iref{intro.execution}, and
 the glvalue can refer to an inactive member of a union\iref{class.union}.
 \end{note}