Add `:nodoc` to `table_structure`

Follow up to #27008.

`table_structure` is an internal method so it is better to hide it in
the doc. And alias `table_structure` to `column_definitions` to remove
the duplicated `columns` method in the sqlite3 adapter.

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -304,14 +304,6 @@ def view_exists?(view_name) # :nodoc:
         select_values(sql, "SCHEMA").any?
       end
 
-      # Returns an array of +Column+ objects for the table specified by +table_name+.
-      def columns(table_name) # :nodoc:
-        table_name = table_name.to_s
-        table_structure(table_name).map do |field|
-          new_column_from_field(table_name, field)
-        end
-      end
-
       def new_column_from_field(table_name, field) # :nondoc:
         case field["dflt_value"]
         when /^null$/i
@@ -434,11 +426,12 @@ def rename_column(table_name, column_name, new_column_name) #:nodoc:
 
       protected
 
-        def table_structure(table_name)
+        def table_structure(table_name) # :nodoc:
           structure = exec_query("PRAGMA table_info(#{quote_table_name(table_name)})", "SCHEMA")
           raise(ActiveRecord::StatementInvalid, "Could not find table '#{table_name}'") if structure.empty?
           table_structure_with_collation(table_name, structure)
         end
+        alias column_definitions table_structure
 
         def alter_table(table_name, options = {}) #:nodoc:
           altered_table_name = "a#{table_name}"