improve eap xPaaS docs on how to enable incremental builds

There is nothing on the EAP xPaaS docs [1] describing how you can enable incremental builds. Today you have to manually change the BuildConfig to include the incremental property like this snippet:

  strategy:     sourceStrategy:       forcePull: true
      from:         kind: ImageStreamTag
        name: 'jboss-eap71-openshift:latest'
        namespace: openshift
      incremental: true

The reference for this on Openshift Docs is [2].

[1] https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.1/html-single/red_hat_jboss_enterprise_application_platform_for_openshift/

[2] https://docs.openshift.com/container-platform/3.9/dev_guide/builds/build_strategies.html#incremental-builds