back fill a breaking doc for yaml parsing
* Build step headers are now sticky, staying at the top of the body on the build page instead of scrolling out of sight as you navigate long logs. This should make it easier to keep track of which step you're inspecting in complicated jobs. #4230
+#### <sub><sup><a name="v550-note-31" href="#v550-note-31">:link:</a></sup></sub> feature, breaking
+
+* YAML parsing across the system has been refactored, and in the process has become stricter. There are now some cases (like in [task configs](https://github.com/concourse/concourse/issues/4512)) where you must use an array of hashes when previously a simple hash would be considered valid. Another example is [task_params](https://concourse-ci.org/tasks.html#task-params) where you must use a single YAML mapping instead of a sequence of mappings. When in doubt, refer to the [docs](https://concourse-ci.org/tasks.html). #3505
+
#### <sub><sup><a name="v550-note-4264" href="#v550-note-4264">:link:</a></sup></sub> fix
* When using the overlay volume driver, you would have to be cautious about preserving your mount table. If it had been cleared, you're volumes would appear empty. Baggageclaim now has the capability to recover missing mounts when the process restarts. #4264
