Update manual.md (#20321)

* Update manual.md

add explanation for $#.

* Update manual.md

add explanation.

Author: foldl

doc/manual.md

@@ -7855,8 +7855,9 @@ The `codegenDecl` pragma can be used to directly influence Nim's code
 generator. It receives a format string that determines how the variable
 or proc is declared in the generated code.
 
-For variables, $1 in the format string represents the type of the variable
-and $2 is the name of the variable.
+For variables, $1 in the format string represents the type of the variable,
+$2 is the name of the variable, and each appearance of $# represents $1/$2
+respectively according to its position.
 
 The following Nim code:
 
@@ -7872,7 +7873,8 @@ will generate this C code:
   ```
 
 For procedures, $1 is the return type of the procedure, $2 is the name of
-the procedure, and $3 is the parameter list.
+the procedure, $3 is the parameter list, and each appearance of $# represents
+$1/$2/$3 respectively according to its position.
 
 The following nim code: