Clean up a few things related to profiles (#8526)

- Greatly expand API docs

- Clean up code in misc ways

  - Instead of a complicated single loop on generations, do different
    operations in successive subsequent steps.

  - Avoid `ref` in one place where `&` is fine

  - Just return path instead of mutating an argument in `makeName`

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libcmd/command.cc

@@ -239,9 +239,7 @@ void MixProfile::updateProfile(const StorePath & storePath)
     if (!store) throw Error("'--profile' is not supported for this Nix store");
     auto profile2 = absPath(*profile);
     switchLink(profile2,
-        createGeneration(
-            ref<LocalFSStore>(store),
-            profile2, storePath));
+        createGeneration(*store, profile2, storePath));
 }
 
 void MixProfile::updateProfile(const BuiltPaths & buildables)