update-api-reports.yml would run yarn build and generate API reports (the md files in common/api-review that we use to check if the API surface has changed) in case the contributor forgot to (it would be run if the contributor runs yarn build locally after making any changes but sometimes people forget).

It also commits the changes to the remote branch, which causes some friction because

1. the contributor's local branch gets out of sync with the remote and they have to make sure to pull before doing anything else on their branch
2. CI won't run on that commit and all the checks will be waiting indefinitely if the contributor doesn't add any more commits after that

Switched it to the way the docs check works, which is to fail and log a message to run yarn build or yarn docgen:all as is appropriate. (If there are differences found in both common/api-review and docs-devsite, then the suggestion is to run yarn docgen:all as it covers both).

Removed the final message step as the workflow log UI seems to direct you to look straight at the step that failed anyway. Put the messages in the failed step.

Tested api-report changes only and tested with docs changes only, and both seem to work.