Document global properties that affect all pipeline uploads to a build

[keithduncan]

We have a couple of global fields in the pipeline YAML: env and agents. These are saved on the build rather than just applying to the steps that are being uploaded.

Several times now customers have been surprised that these not only affect the steps in the current pipeline upload of steps, but all subsequently uploaded steps too.

e.g.

cat <<EOF
agents:
   queue: my-queue
steps:
  - command: echo hello
EOF | buildkite-agent pipeline upload

cat <<EOF
agents:
   foo: bar
steps:
  - command: echo hello
EOF | buildkite-agent pipeline upload

The second upload’s step will look for an agent whose query rules are queue:my-queue,foo:bar when such an agent might not exist.

The same thing happens for env. Customers have been surprised to find that these are saved 'global' to the build and are applied to subsequent steps.

In addition to documentation, I think this should be surfaced in the product in the Timeline tab of the affected jobs, to show that any build global agents and env state was merged in to their step-specific config.