[over.built] Improve placeholder phrasing

Author: tkoeppe

source/overloading.tex

@@ -3737,8 +3737,9 @@
 \end{codeblock}
 
 \pnum
-For every pointer to member type \tcode{\placeholder{T}} or type \tcode{std::nullptr_t} there
-exist candidate operator functions of the form
+For every \tcode{\placeholder{T}}, where \tcode{\placeholder{T}}
+is a pointer to member type or \tcode{std::nullptr_t},
+there exist candidate operator functions of the form
 
 \begin{codeblock}
 bool    operator==(@\placeholder{T}@, @\placeholder{T}@);