Add comments to fulcrum.nix

system/bitcoin/fulcrum.nix

@@ -1,5 +1,14 @@
+### FULCRUM
 {
   services.fulcrum = {
+    # Set this to enable fulcrum, an Electrum server implemented in C++.
+    #
+    # Compared to electrs, fulcrum has higher storage demands but
+    # can serve arbitrary address queries instantly.
+    #
+    # Before enabling fulcrum, and for more info on storage demands,
+    # see the description of option `enable` in ../modules/fulcrum.nix
+    #
     enable = true;
   };
 }