Document immediate_hydration as Pro-only in react_on_rails_pro.rb

Per code review feedback, immediate_hydration should be configured in the
Pro initializer, not the base initializer:

1. Updated base initializer template to include Pro features section
   - Documents that immediate_hydration requires Pro
   - Directs users to configure in react_on_rails_pro.rb
   - Provides link to Pro information

2. Updated docs/api-reference/configuration.md
   - Removed immediate_hydration from base config example
   - Added Pro features section with clear example
   - Shows proper usage in ReactOnRailsPro.configure block

This ensures users understand immediate_hydration is Pro-only and know
where to configure it properly.

Author: justin808

docs/api-reference/configuration.md

@@ -279,11 +279,22 @@ ReactOnRails.configure do |config|
   # See [16.0.0 Release Notes](docs/release-notes/16.0.0.md) for more details.
   # config.defer_generated_component_packs = false
 
-  # Default is false
-  # React on Rails Pro (licensed) feature: When true, components hydrate immediately as soon as
-  # their server-rendered HTML reaches the client, without waiting for the full page load.
-  # This improves time-to-interactive performance.
-  config.immediate_hydration = false
+  ################################################################################
+  # REACT ON RAILS PRO FEATURES
+  ################################################################################
+  # The following features require React on Rails Pro and should be configured
+  # in config/initializers/react_on_rails_pro.rb using ReactOnRailsPro.configure:
+  #
+  # - immediate_hydration: When true, components hydrate immediately as soon as
+  #   their server-rendered HTML reaches the client, without waiting for the full page load.
+  #   This improves time-to-interactive performance.
+  #
+  # Example (in config/initializers/react_on_rails_pro.rb):
+  #   ReactOnRailsPro.configure do |config|
+  #     config.immediate_hydration = true
+  #   end
+  #
+  # For more information, visit: https://www.shakacode.com/react-on-rails-pro
 
   ################################################################################
   # I18N OPTIONS

lib/generators/react_on_rails/templates/base/base/config/initializers/react_on_rails.rb.tt

@@ -78,4 +78,17 @@ ReactOnRails.configure do |config|
   # Default is :defer which provides good performance and prevents component registration race conditions.
   # For React on Rails Pro users, :async can provide better performance by loading scripts asynchronously.
   config.generated_component_packs_loading_strategy = :defer
+
+  ################################################################################
+  # REACT ON RAILS PRO FEATURES
+  ################################################################################
+  # The following features require React on Rails Pro and should be configured
+  # in config/initializers/react_on_rails_pro.rb:
+  #
+  # - immediate_hydration: Enables components to hydrate immediately as their
+  #   server-rendered HTML reaches the client, improving time-to-interactive.
+  #   Configure in ReactOnRailsPro.configure block.
+  #
+  # For more information on React on Rails Pro features, visit:
+  # https://www.shakacode.com/react-on-rails-pro
 end