Add 'nix upgrade-nix' manpage

src/nix/upgrade-nix.cc

@@ -37,18 +37,11 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
         return "upgrade Nix to the latest stable version";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
-            Example{
-                "To upgrade Nix to the latest stable version:",
-                "nix upgrade-nix"
-            },
-            Example{
-                "To upgrade Nix in a specific profile:",
-                "nix upgrade-nix -p /nix/var/nix/profiles/per-user/alice/profile"
-            },
-        };
+        return
+          #include "upgrade-nix.md"
+          ;
     }
 
     Category category() override { return catNixInstallation; }

src/nix/upgrade-nix.md

@@ -0,0 +1,28 @@
+R""(
+
+# Examples
+
+* Upgrade Nix to the latest stable version:
+
+  ```console
+  # nix upgrade-nix
+  ```
+
+* Upgrade Nix in a specific profile:
+
+  ```console
+  # nix upgrade-nix -p /nix/var/nix/profiles/per-user/alice/profile
+  ```
+
+# Description
+
+This command upgrades Nix to the latest version. By default, it
+locates the directory containing the `nix` binary in the `$PATH`
+environment variable. If that directory is a Nix profile, it will
+upgrade the `nix` package in that profile to the latest stable binary
+release.
+
+You cannot use this command to upgrade Nix in the system profile of a
+NixOS system (that is, if `nix` is found in `/run/current-system`).
+
+)""