[Doc] There should be a reminder for PostBuildCleanup's position

[mszhanyi]

Describe the context

• Extension: PostBuildCleanup

Describe the problem and expected behavior Generally, users like to set the PostBuildCleanup as the last step of the job. But if other task defined Post-Jobs and the Post-Jobs would use some build files, like Cache@2. The workflow would fail because the PostBuildCleanup has clean all files.

So, the documentation should remind user this risk and provide the workaround like https://github.com/MicrosoftPremier/VstsExtensions/issues/193#issuecomment-1256818681.

[ReneSchumacher]

This is addressed in the docs (we recommend it as the first task). In addition, we provided guidance esp. for YAML pipelines on how to replace our task with custom scripting.