Add 'nix store ping' manpage

src/nix/ping-store.cc

@@ -8,17 +8,14 @@ struct CmdPingStore : StoreCommand
 {
     std::string description() override
     {
-        return "test whether a store can be opened";
+        return "test whether a store can be accessed";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
-            Example{
-                "To test whether connecting to a remote Nix store via SSH works:",
-                "nix store ping --store ssh://mac1"
-            },
-        };
+        return
+          #include "ping-store.md"
+          ;
     }
 
     void run(ref<Store> store) override

src/nix/ping-store.md

@@ -0,0 +1,30 @@
+R""(
+
+# Examples
+
+* Test whether connecting to a remote Nix store via SSH works:
+
+  ```console
+  # nix store ping --store ssh://mac1
+  ```
+
+* Test whether a URL is a valid binary cache:
+
+  ```console
+  # nix store ping --store https://cache.nixos.org
+  ```
+
+* Test whether the Nix daemon is up and running:
+
+  ```console
+  # nix store ping --store daemon
+  ```
+
+# Description
+
+This command tests whether a particular Nix store (specified by the
+argument `--store` *url*) can be accessed. What this means is
+dependent on the type of the store. For instance, for an SSH store it
+means that Nix can connect to the specified machine.
+
+)""