chore: Update language onboarding guidelines to include container runtime specs (#2633)

fixes #2611

---------

Signed-off-by: ldetmer <[email protected]>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Jon Skeet <[email protected]>

Author: 3 people

doc/language-onboarding.md

@@ -57,6 +57,13 @@ need to be executable by any user ID within the container.
 * Create a docker file for your container [example](https://github.com/googleapis/google-cloud-go/blob/main/internal/librariangen/Dockerfile)
 * Create a cloudbuild file [example](https://github.com/googleapis/google-cloud-go/blob/main/internal/librariangen/cloudbuild-exitgate.yaml) that uploads your image to us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev
 
+### Guidelines on Language Container Runtimes
+
+You should be able to run the `generate` or `release-init` commands for an API such as Google Cloud Functions in less than a
+minute. We understand that some libraries may take longer to process, however, long runtimes can adversely affect your
+ability to roll out emergency changes. While the CLI typically calls the container only for libraries with changes, a
+generator update could trigger a run for all your libraries.
+
 ### Implement Container Contracts
 
 The following sections detail the contracts for each container command.