Add 'nix store diff-closures' manpage

src/nix/diff-closures.cc

@@ -121,14 +121,11 @@ struct CmdDiffClosures : SourceExprCommand
         return "show what packages and versions were added and removed between two closures";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
-            {
-                "To show what got added and removed between two versions of the NixOS system profile:",
-                "nix store diff-closures /nix/var/nix/profiles/system-655-link /nix/var/nix/profiles/system-658-link",
-            },
-        };
+        return
+          #include "diff-closures.md"
+          ;
     }
 
     void run(ref<Store> store) override

src/nix/diff-closures.md

@@ -0,0 +1,51 @@
+R""(
+
+# Examples
+
+* Show what got added and removed between two versions of the NixOS
+  system profile:
+
+  ```console
+  # nix store diff-closures /nix/var/nix/profiles/system-655-link /nix/var/nix/profiles/system-658-link
+  acpi-call: 2020-04-07-5.8.16 → 2020-04-07-5.8.18
+  baloo-widgets: 20.08.1 → 20.08.2
+  bluez-qt: +12.6 KiB
+  dolphin: 20.08.1 → 20.08.2, +13.9 KiB
+  kdeconnect: 20.08.2 → ∅, -6597.8 KiB
+  kdeconnect-kde: ∅ → 20.08.2, +6599.7 KiB
+  …
+  ```
+
+# Description
+
+This command shows the differences between the two closures *before*
+and *after* with respect to the addition, removal, or version change
+of packages, as well as changes in store path sizes.
+
+For each package name in the two closures (where a package name is
+defined as the name component of a store path excluding the version),
+if there is a change in the set of versions of the package, or a
+change in the size of the store paths of more than 8 KiB, it prints a
+line like this:
+
+```console
+dolphin: 20.08.1 → 20.08.2, +13.9 KiB
+```
+
+No size change is shown if it's below the threshold. If the package
+does not exist in either the *before* or *after* closures, it is
+represented using `∅` (empty set) on the appropriate side of the
+arrow. If a package has an empty version string, the version is
+rendered as `ε` (epsilon).
+
+There may be multiple versions of a package in each closure. In that
+case, only the changed versions are shown. Thus,
+
+```console
+libfoo: 1.2, 1.3 → 1.4
+```
+
+leaves open the possibility that there are other versions (e.g. `1.1`)
+that exist in both closures.
+
+)""