Document waiting

[holmanb]

• I have signed the CLA: https://ubuntu.com/legal/contributors
• I have included a comprehensive commit message using the guide below
• I have added unit tests to cover the new behavior under tests/unittests/
• I have kept the change small, avoiding unnecessary whitespace or non-functional changes.
• I have added a reference to issues that this PR relates to in the PR message (Refs integration: do not LXD bind mount /etc/cloud/cloud.cfg.d #1234, Fixes integration: do not LXD bind mount /etc/cloud/cloud.cfg.d #1234)
• I have updated the documentation with the changed behavior.

Context

#6286 (comment)

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[TheRealFalcon]

Content is good. We generally use double backticks for the inline callouts instead of single backticks, so I think it'd be better to do that for consistency.

[holmanb]

Thanks for the review @TheRealFalcon. I think that I've addressed everything.

[blackboxsw]

Content is good. We generally use double backticks for the inline callouts instead of single backticks, so I think it'd be better to do that for consistency.

Yep, single backticks is italics double is bold. And italics are hard to read/see/differentiate in our RTD theme (and for those with significant dyslexia).

[TheRealFalcon]

LGTM!

[TheRealFalcon]

@holmanb , since you specified rebase, I'm leaving it up to you to merge.

[Jalle19]

It should perhaps be noted that if you have two services, A and B, where A depends on B, and B needs to wait for cloud-init, you have to add After=multi-user.target cloud-final.service to both A and B. It's not enough to have simply After=B.service in service A, since it will still be adding that undocumented Before=multi-user.target dependency due to it having WantedBy=multi-user.target.

I really don't understand why cloud-final.service runs after multi-user.target. Standard practice is that the system is completely started when multi-user.target is reached. That's no longer the case when cloud-init is involved.