The default value for --platform is not documented

[MikeSpreitzer]

I was not the first person (see #946) to be surprised by the fact that ko build defaults to building for linux/amd64 regardless of the host platform. The documentation for ko build should explicitly state what is the default value for --platform.

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.