Add description for db:create and db:drop test tasks in AR

This will help identify these tasks exists, since they are easier to
setup things for running all AR tests.

activerecord/Rakefile

@@ -39,8 +39,10 @@ namespace :test do
 end
 
 namespace :db do
-  task :create => ['mysql:build_databases', 'postgresql:build_databases']
-  task :drop => ['mysql:drop_databases', 'postgresql:drop_databases']
+  desc 'Build MySQL and PostgreSQL test databases'
+  task create: ['mysql:build_databases', 'postgresql:build_databases']
+  desc 'Drop MySQL and PostgreSQL test databases'
+  task drop: ['mysql:drop_databases', 'postgresql:drop_databases']
 end
 
 %w( mysql mysql2 postgresql sqlite3 sqlite3_mem firebird db2 oracle sybase openbase frontbase jdbcmysql jdbcpostgresql jdbcsqlite3 jdbcderby jdbch2 jdbchsqldb ).each do |adapter|