Expanded upon directory exclusion for clarification along with wiki info

What does this MR do?

This MR seeks to remove some uncertainty based on the language used in the backup_restore.md docs regarding the SKIP variable. The header of that section talks about excluding aspects while the main sentence talks about how to set what you can backup. It's a small change but can remove all confusion from the doc. I had to test the backup process myself because I had a seed of doubt in regards to the process.

It also adds a small bit of info on how wikis are handled in an backup and what category they are a child of.

Related issues

None that could be found.

Closes

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ee/development/documentation/#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to the old docs in the GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• If working on CE and the ee-compat-check jobs fails, submit an MR to EE with the changes as well.
• Ping one of the technical writers for review.