fix decode multi YAML doc

[xuzhenglun]

What this PR does / why we need it:

previous doc
--- # THIS IS A COMMENT
---   
really doc here

below YAML separator is valid for kubectl but does not work in this repo.

1. the first separator has some comments.
2. the second separator has white space at the end of it.

In current behavior, really doc here will treat as a part of previous doc, and really doc here will be eaten in yaml.Unmarshal later.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Additional documentation:

[k8s-ci-robot]

Hi @xuzhenglun. Thanks for your PR.

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[atoato88]

/ok-to-test

Thank you to create, and I'm glad that this PR has unit test! 😄
It help to understand the update.

[atoato88]

/lgtm

[xuzhenglun]

@atoato88

shall we make this progress go-ahead?

[justinsb]

Sorry for delay and thanks for fixing this!

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justinsb, xuzhenglun

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