Request to refine the description of GitLab credential in "Create Pipeline" modal

**What would you like to be added**:

Request to refine the description of GitLab credential in "Create Pipeline" modal, please see the screenshot below.


I'm not sure how this description should be defined. If anyone has a better suggestion, I hope they can put forward them.

**Why is this needed**:

> [Optional] Add Checkout Credentials (SSHPrivateKey or Username/Password) if there is any private projects that will be built by the plugin.
> https://github.com/jenkinsci/gitlab-branch-source-plugin#multibranch-pipeline-jobs

In fact, the credential is an optional parameter, being used to checkout source code instead of for GitLab API connection. So this credential cannot be used to list GitLab private repository here.

/area devops
/priority low
/cc @kubesphere/sig-devops

Start with the Create Pipeline modal described in the issue and compare its GitLab credential text with the linked Jenkins GitLab Branch Source guidance. Confirm that the credential is optional checkout authentication rather than GitLab API access, then define and verify revised wording in that modal.

Written by the indexing model from the issue text.