docs: Add examples for step update commands for priority and notify attributes

[tomowatt]

Description

buildkite-agent step update command has the ability to update both the priority and notify attributes on Steps but these were not documented so adding examples of them.
Docs PR to follow when merged in.

Context

Changes

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

Disclosures / Credits

Came across the additional usage of updating priority and notify attributes by pure chance when investigating a ticket.

[DrJosh9000]

Thanks