prow: document default preset behaviour

[cblecker]

What should be cleaned up or changed:
prow's preset functionality has the ability to have one or more "default" presets if you omit the labels field on a specific preset in the config.

example:

presets:
- env:
  - name: GOPROXY
    value: "https://proxy.golang.org"

This sets this env var for all jobs.

We need the following documentation locations updated:

• https://github.com/kubernetes/test-infra/blob/master/prow/jobs.md#presets - ensure an example of defaulting behaviour is documented
• https://github.com/kubernetes/test-infra/tree/master/config/jobs#job-presets - link to the default we use and mention it sets GOPROXY by default

We also need to ensure that there is sufficient test coverage for this use case (may or may not need additional test cases).

Provide any links for context:
#13648

[cblecker]

/assign @spiffxp

[spiffxp]

/milestone v1.16
/area jobs
/area prow
/remove-kind cleanup
/kind documentation

[spiffxp]

/sig testing