Extend internal API docs, part 2

Picking up from #8111.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libcmd/command-installable-value.hh

@@ -3,8 +3,15 @@
 
 namespace nix {
 
+/**
+ * An InstallableCommand where the single positional argument must be an
+ * InstallableValue in particular.
+ */
 struct InstallableValueCommand : InstallableCommand
 {
+    /**
+     * Entry point to this command
+     */
     virtual void run(ref<Store> store, ref<InstallableValue> installable) = 0;
 
     void run(ref<Store> store, ref<Installable> installable) override;