operations diagram: store' to the right

this also looks more diverse, hopefully easier to distinguish

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

doc/manual/src/architecture/store/store.md

@@ -26,14 +26,20 @@ A Nix store can *add*, *retrieve*, and *delete* store objects.
                 [ data ]
                     |
                     V
-    [ store ] ---> add ----> [ store' ], [ reference ]
+    [ store ] ---> add ----> [ store' ]
+                    |
+                    V
+              [ reference ]
 
 <!-- -->
 
               [ reference ]
                     |
                     V
-    [ store ] ---> get ----> [ store object ]
+    [ store ] ---> get
+                    |
+                    V
+             [ store object ]
 
 <!-- -->
 
@@ -49,7 +55,12 @@ It can *perform builds*, that is, create new store objects by transforming build
               [ reference ]
                     |
                     V
-    [ store ] --> build --(maybe)--> [ store' ], [ reference' ]
+    [ store ] --> build
+                       \
+                      (maybe) --> [ store' ]
+                         |
+                         V
+                   [ reference ]
 
 
 As it keeps track of references, it can [garbage-collect][garbage-collection] unused store objects.