Update exit code docs

[MichaelEischer]

What does this PR change? What problem does it solve?

See the first commit for the main changes. The docs for the backup command now link to the scripting section. I've kept the exit status codes section in the backup section as the descriptions there focus one the backup command such that the exact same text wouldn't fit into the scripting section.

Was the change previously discussed in an issue or on the forum?

No.

Checklist

• [x] I have read the contribution guidelines.
• [x] I have enabled maintainer edits.
• ~~[ ] I have added tests for all code changes.~~
• [x] I have added documentation for relevant changes (in the manual).
• ~~[ ] There's a new file in changelog/unreleased/ that describes the changes for our users (see template).~~
• [x] I have run gofmt on the code in all commits.
• [x] All commit messages are formatted in the same style as the other commits in the repo.
• [x] I'm done! This pull request is ready for review.