clarify *wrapper task* and *task variable*

[lmmarsano]

The API documentation mentions wrapper task and task variable

azure-pipelines-task-lib/node/docs/azure-pipelines-task-lib.md

Line 327 in c118389

Gets a variable value that is set by previous step from the same wrapper task.

I've seen regular tasks before.
I've seen pipeline variables.
However, I've never seen the other terms in that combination defined in documentation here or elsewhere.
The simplistic custom task tutorial doesn't mention anything about wrapper tasks.

Please link to proper documentation defining and explaining those terms or do so in your documentation.
What are they?
How do we create them?

[github-actions]

This issue has had no activity in 90 days. Please comment if it is not actually stale

[lmmarsano]

Not stale.

[github-actions]

This issue has had no activity in 90 days. Please comment if it is not actually stale