Follow-up: evaluate the 'out of storage' banner text

In #323968 (closed) we updated the out of storage banner text, however we need to evaluate the text if it needs any update.

If it's good as it's we should evaluate adding a link to the docs where the text says:

To learn more about reducing storage capacity please visit our docs

The link could be pointing to: https://docs.gitlab.com/ee/user/usage_quotas.html