Update installation and update docs for 11 4
What does this MR do?
This updates the documentation for updating/installing GitLab 11.3
What are the relevant issue numbers?
This is a step in gitlab-org/release/tasks#460 (closed)
Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary -
Documentation created/updated -
Tests added for this feature/bug -
Conforms to the code review guidelines -
Conforms to the merge request performance guidelines -
Conforms to the style guides -
Conforms to the database guides