Check and clarify `global.serviceAccount` usage

What does this MR do?

Docs: Clarify `global.serviceAccount` usage

Clarifies how to use `global.serviceAccount keys`:

* Clarifies the description of each key.
* Adds a note about not using `create=true` with a global name.
* Adds a checkConfig entry to help users avoid the misconfiguration
  above.
* Adds tests to confirm the checkConfig entry works as expected.

Changelog: other

Related issues

Closes #4540 (closed)

Test plan

$ bundle exec rspec -c -f d spec/integration/check_config_spec.rb:45
warning: parser/current is loading parser/ruby27, which recognizes2.7.6-compliant syntax, but you are running 2.7.5.
Please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.
Excluding specs that require access to k8s cluster
Run options:
  include {:focus=>true, :locations=>{"./spec/integration/check_config_spec.rb"=>[45]}}
  exclude {:type=>"feature"}

checkConfig template
  serviceAccount
    when global ServiceAccount name is provided with `create=false`
      succeeds
    when global ServiceAccount name is provided with `create=true`
      returns an error

Finished in 5.49 seconds (files took 0.67716 seconds to load)
2 examples, 0 failures

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.