docs: adding documentation to add an array element using overrides

[ArchangelX360]

What this PR does / why we need it:

Adding a value to an array could be confusing for users, an example in the docs could alleviate the confusion.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1224

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[xunpan]

@ArchangelX360
It is better to have a valid option for the case to me. bar seems not valid for nginx.

[ArchangelX360]

@xunpan agreed good catch, -q should be.

[xunpan]

/lgtm

[irfanurrehman]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ArchangelX360, irfanurrehman

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