[functional.{syn,bind.place}] Use \vdots; add missing \placeholder (#7723)

Author: lprv

source/utilities.tex

@@ -10798,9 +10798,7 @@
     // \tcode{\placeholder{M}} is the \impldef{number of placeholders for bind expressions} number of placeholders
     @\seebelownc@ _1;                                                                   // freestanding
     @\seebelownc@ _2;                                                                   // freestanding
-               .
-               .
-               .
+               @\vdots@
     @\seebelownc@ _@\placeholdernc{M}@;                                                                   // freestanding
   }
 
@@ -12977,13 +12975,11 @@
 % "namespace placeholders" in [functional.syn]?
 \begin{codeblock}
 namespace std::placeholders {
-  // M is the number of placeholders
+  // \tcode{\placeholder{M}} is the number of placeholders
   @\seebelow@ _1;
   @\seebelow@ _2;
-              .
-              .
-              .
-  @\seebelow@ _M;
+             @\itcorr\vdots@
+  @\seebelow@ _@\placeholdernc{M}@;
 }
 \end{codeblock}