Ensure all headers have #pragma once and are in API docs

`///@file` makes them show up in the internal API dos. A tiny few were
missing `#pragma once`.

src/libcmd/command-installable-value.hh

@@ -1,3 +1,6 @@
+#pragma once
+///@file
+
 #include "installable-value.hh"
 #include "command.hh"
 

src/libcmd/command.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "installable-value.hh"
 #include "args.hh"

src/libcmd/common-eval-args.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "args.hh"
 

src/libcmd/editor-for.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "types.hh"
 

src/libcmd/installable-attr-path.hh

@@ -1,3 +1,6 @@
+#pragma once
+///@file
+
 #include "globals.hh"
 #include "installable-value.hh"
 #include "outputs-spec.hh"

src/libcmd/installable-derived-path.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "installables.hh"
 

src/libcmd/installable-flake.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "installable-value.hh"
 

src/libcmd/installable-value.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "installables.hh"
 #include "flake/flake.hh"

src/libcmd/installables.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "util.hh"
 #include "path.hh"

src/libcmd/legacy.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include <functional>
 #include <map>

src/libcmd/markdown.hh

@@ -1,3 +1,6 @@
+#pragma once
+///@file
+
 #include "types.hh"
 
 namespace nix {

src/libcmd/repl.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "eval.hh"