Add nodoc to `migrations_paths` and `migration_context` in `AbstractAdapter`

These are internally used only.

[ci skip]

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -119,11 +119,11 @@ def initialize(connection, logger = nil, config = {}) # :nodoc:
         end
       end
 
-      def migrations_paths
+      def migrations_paths # :nodoc:
         @config[:migrations_paths] || Migrator.migrations_paths
       end
 
-      def migration_context
+      def migration_context # :nodoc:
         MigrationContext.new(migrations_paths)
       end