The documentation [1] specifies "says "Optionally, specify the ref field to check out a specific Git reference." 
Perhaps it should mention it's only optional if the default branch is master. 

And section [2] should show an example of the payload GitHub sends, and mention the ref field is relevant. i.e., the build will only be triggered in the ref from the github webhook event matches the ref in the git source

[1] https://docs.openshift.com/container-platform/4.13/cicd/builds/creating-build-inputs.html#builds-source-code_creating-build-inputs
[2] https://docs.openshift.com/container-platform/4.13/cicd/builds/triggering-builds-build-hooks.html#builds-using-github-webhooks_triggering-builds-build-hooks