This meta-issue lists all the big changes needed to upgrade the docs to Symfony 4.0:

• Replacekernel.root_dir bykernel.project_dir everywhere. Never explainkernel.root_dir but maybe add a note mentioning thatproject_dir didn't exist before 4.x and people usedroot_dir.
• The file extension of YAML files is now.yaml instead of.yml
• In YAML files, null values are represented asnull instead of~ (change was reverted)
• Application code is no longer stored in AppBundle or any other bundle. Bundles for own code are discouraged.
• Lots of components/bundles (e.g. Symfony Console or MonologBundle) are not installed by default. We should add notes telling users to do:composer req console, etc.
• We need to decide what to do with Symfony Flex aliases. For example, which command should we use to install the Symfony Console package?composer req cli,composer req console,composer require console,composer require symfony/console, etc.
• Most config options are disabled by default. For example, thesession service is not available by default becausesession config is commented inframework.yaml
• SeeRemove Debugging Guide #8538 - make sure we mention all of the debugging commands somewhere
• Commands are now always services, being in aCommand/ directory is not important
• remove references to symfony/framework-standard-edition
• web/ topublic/ (also seeFix server configuration for Symfony 4 #8693 (comment))
• _controller ->controller

Remove deprecated features:

• removed autowiring services based on the types they implement
• removed theDefinitionDecorator class, replaced byChildDefinition
• removed theAutowireServiceResource class and relatedAutowirePass::createResourceForClass() method
• removedLoggingFormatter,Compiler::getLoggingFormatter() andaddLogMessage() class and methods, use theContainerBuilder::log() method instead
• removedFactoryReturnTypePass
• removedContainerBuilder::addClassResource(), use theaddObjectResource() or thegetReflectionClass() method instead.
• removed support for top-level anonymous services
• removed silent behavior for unused attributes and elements
• Removed the "framework.validation.cache" configuration option. Configure the "cache.validator" service under "framework.cache.pools" instead.
• Support for stacked errors in theErrorHandler is deprecated and will be removed in Symfony 4.0.