C API: (breaking) remove nix-flake-c global init

doc/manual/rl-next/c-api-flake-init.md

@@ -0,0 +1,20 @@
+---
+synopsis: C API `nix_flake_init_global` removed
+prs: 12759
+issues: 5638
+---
+
+In order to improve the modularity of the code base, we are removing a use of global state, and therefore the `nix_flake_init_global` function.
+
+Instead, use `nix_flake_settings_add_to_eval_state_builder`. For example:
+
+```diff
+-    nix_flake_init_global(ctx, settings);
+-    HANDLE_ERROR(ctx);
+-
+     nix_eval_state_builder * builder = nix_eval_state_builder_new(ctx, store);
+     HANDLE_ERROR(ctx);
+ 
++    nix_flake_settings_add_to_eval_state_builder(ctx, settings, builder);
++    HANDLE_ERROR(ctx);
+```