lua-shared-dicts improvements, fixes and documentation

[ElvinEfendi]

What this PR does / why we need it:

• Document the setting
• Make sure we ignore the value when it is not an integer
• Add more validation on the number of dictionaries and max size
• Add more realistic test cases

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

Special notes for your reviewer:

[ElvinEfendi]

/test pull-ingress-nginx-test

[ElvinEfendi]

/retest

[aledbf]

@ElvinEfendi can you adjust the size of the /configuration location using the sizes?
https://github.com/kubernetes/ingress-nginx/blob/master/rootfs/etc/nginx/template/nginx.tmpl#L584

[aledbf]

/retest

[ElvinEfendi]

@ElvinEfendi can you adjust the size of the /configuration location using the sizes?

@aledbf done: https://github.com/kubernetes/ingress-nginx/pull/4446/files#diff-980db9e4b88704f12338bd074839f94eR584

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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