chore(doc): add instruction to trigger container test from update-image with flag (#2826)

Update instruction on container test and include link to pkg
documentation.

Fixes #2621

Author: zhumin8

doc/repository-library-onboarding.md

@@ -7,7 +7,7 @@ This guide should be followed when onboarding new repositories/libraries.
 2) Add `.librarian` directory to your repository with appropriate configuration files. See details [here](https://github.com/googleapis/librarian/blob/main/doc/language-onboarding.md#configure-repository-to-work-with-librarian-cli)
 3) You should only start with 1 library to validate the flow (follow instructions below)
 4) If your repository contains multiple libraries, start ramping up slowly until all libraries are in your `state.yaml` file and have migrated to librarian.
-5) To complete onboarding you should run the librarian test-container generate command to validate that all libraries are getting generated correctly. Note this command is WIP, [documentation will be added](https://github.com/googleapis/librarian/issues/2621) once available.  
+5) To complete onboarding you should run the librarian test-container generate command to validate that all libraries are getting generated correctly. Note this standalone command is WIP, currently you can run [update-image](https://pkg.go.dev/github.com/googleapis/librarian/cmd/librarian#hdr-update_image) command with `-test` flag to trigger tests after generation.  
 6) To correctly parse the commit message of a merge commit, only allow squash merging
 and set the default commit message to **Pull request title and description**.
 ![Pull request settings](assets/setting-pull-requests.webp)