Add documentation for inactive users in the User cohort

What does this MR do?

Closes #31195 (closed)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

added 1 commit

• 889b7e82 - Fix headings

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assigned to @smcgivern

@regisF thanks, one question!

changed milestone to %9.1

assigned to @regisF

resolved all discussions

added 1 commit

• 650eed6a - Fixed wording

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approved this merge request

enabled an automatic merge when the pipeline for 650eed6a succeeds

@godfat @felipe_artur this documentation-only change should be in 9.1.0.

merged

mentioned in commit 9c59667f

The stable branch doesn't have this file. So picking this would add the entire file.

Picked into 9-1-stable, will go into 9.1.0-rc6

@godfat this builds off https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10481, so that should have been picked first. Sorry, I should have mentioned

Just realized that the file was added in !10481 (merged), and that's also the only conflict picking it!

I'll use the content in this merge request to resolve the conflict.

@smcgivern It's ok! I realized that soon after I picked it :P

mentioned in commit 5c023701