Clarify STI models loading process with a comment.

config/initializers/sti_eager_load.rb

@@ -28,16 +28,21 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
+# Here we are loading STI models explicitly using rails autoloader.
+# It is relevant for environments where lazy loading is enabled (usually, development and testing).
+# If an STI model has not been loaded it can lead to undesired behavior like:
+# Fetching not loaded yet STI model (ServiceAccount) through its parent model(User)
+# (e.g. User.find(service_account_id) raises ActiveRecord::NotFound.
 Rails.application.config.to_prepare do
-  # Load Enumeration descendants due to STI
+  # Load Enumeration descendants
   IssuePriority
 
-  # Load Principal descendants due to STI
+  # Load Principal descendants
   User
   PlaceholderUser
   Group
 
-  # Use User descendants due to STI
+  # Load User descendants
   SystemUser
   AnonymousUser
   Users::InexistentUser