Start to document the local-overlay store

src/libstore/local-overlay-store.cc

@@ -5,6 +5,13 @@
 
 namespace nix {
 
+std::string LocalOverlayStoreConfig::doc()
+{
+    return
+        #include "local-overlay-store.md"
+        ;
+}
+
 Path LocalOverlayStoreConfig::toUpperPath(const StorePath & path) {
     return upperLayer + "/" + path.to_string();
 }

src/libstore/local-overlay-store.hh

@@ -61,14 +61,7 @@ struct LocalOverlayStoreConfig : virtual LocalStoreConfig
         return ExperimentalFeature::LocalOverlayStore;
     }
 
-    std::string doc() override
-    {
-        return
-          ""
-          // FIXME write docs
-          //#include "local-overlay-store.md"
-          ;
-    }
+    std::string doc() override;
 
     /**
      * Given a store path, get its location (if it is exists) in the

src/libstore/local-overlay-store.md

@@ -0,0 +1,7 @@
+R"(
+
+**Store URL format**: `local-overlay`
+
+This store type is a variation of the [local store](#local-store) designed to leverage overlayfs.
+
+)"