Clarify how concurrency works for dynamic jobs

The [concurrency groups documentation][1] currently says:

> As pipeline uploads add more jobs to the build after it has started, the jobs that they add will inherit the creation time of the pipeline upload rather than the build.

The way this is worded still leaves me wondering what the creation time of a dynamically uploaded job will be. I could interpret it as either:

(a) The time at which the `buildkite-agent pipeline upload` command runs.
(b) The time at which the step containing the `buildkite-agent pipeline upload` command is created.

In matter of fact, I'm unsure which it is. Perhaps adding an illustrative example would help?

[1]: https://buildkite.com/docs/pipelines/controlling-concurrency