reword notes on copying

clarify that we are copying between different stores. we have not
introduced that notion or why it would be interesting, but for now it
should be fine to keep it in context of the store directory.

we could move that later to a more detailed explanation of different
store types.

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

@@ -42,7 +42,7 @@ It is important which store a given store object belongs to:
 Files in the store object can contain store paths, and processes may read these paths.
 Nix can only guarantee [referential integrity](store.md#closure) if store paths do not cross store boundaries.
 
-Therefore one can only copy store objects if
+Therefore one can only copy store objects to a different store if
 
 - the source and target stores' directories match
 
@@ -50,8 +50,9 @@ Therefore one can only copy store objects if
 
 - the store object in question has no references, that is, contains no store paths.
 
-To move a store object to a store with a different store directory, it has to be rebuilt, together with all its dependencies.
+One cannot copy a store object to a store with a different store directory.
-It is in general not enough to replace the store directory string in file contents, as this may break internal offsets or content hashes.
+Instead, it has to be rebuilt, together with all its dependencies.
+It is in general not enough to replace the store directory string in file contents, as this may render executables unusable by invalidating their internal offsets or checksums.
 
 # Digest {#digest}