[expr.add] Simplify phrasing around ptrdiff_t and avoid redundancy

Author: jensmaurer

source/expressions.tex

@@ -7201,15 +7201,10 @@
 \end{note}
 
 \pnum
-\indextext{\idxcode{ptrdiff_t}!implementation-defined type of}%
-\indextext{subtraction!implementation-defined pointer}%
 \indextext{\idxcode{ptrdiff_t}}%
 \indextext{comparison!undefined pointer}%
-When two pointer expressions \tcode{P} and \tcode{Q} are subtracted,
-the type of the result is an \impldef{type of \tcode{ptrdiff_t}} signed
-integral type; this type shall be the same type that is named by
-\tcode{std::ptrdiff_t} in the \libheader{cstddef}
-header\iref{support.types.layout}.
+The result of subtracting two pointer expressions \tcode{P} and \tcode{Q}
+is a prvalue of type \tcode{std::ptrdiff_t}\iref{support.types.layout}.
 \begin{itemize}
 \item If \tcode{P} and \tcode{Q} both evaluate to null pointer values,
 the result is 0.