Add some docs about the volcano job plugins

[Thor-wl]

In order to make it clear for users, suggest to add more docs about the volcano job. The docs should give more details about user guidance.

• env ( add user guidance of env plugin #2153 )
• ssh (add user guidance for ssh plugin #2168)
• svc ( add user guidance for svc plugin #2162 )

[Thor-wl]

/assign

[Thor-wl]

/close

[volcano-sh-bot]

@Thor-wl: Closing this issue.

In response to this:

/close

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.