email publisher: Clarify format of recipients list in docstring
email publisher: Clarify format of recipients list in docstring

The list of recipients should be a space separated list. This is
obvious when looking at the example, but not from the description
of the parameter.

Add clarification in the docstring.

Change-Id: I59d186adc70dbb9dec042ebb75d58f7fbb17163a

wikimedia/integration-jenkins-job-builder,wsoula/jenkins-job-builder,phinexus/jenkins-job-builder,phinexus/jenkins-job-builder,gforcada/jenkins-job-builder,wsoula/jenkins-job-builder,lukas-bednar/jenkins-job-builder,rsig/jenkins-job-builder,OSSystems/jenkins-job-builder,phinexus/jenkins-job-builder,gforcada/jenkins-job-builder,lukas-bednar/jenkins-job-builder,rsig/jenkins-job-builder,tamac-io/jenkins-job-builder,OSSystems/jenkins-job-builder,joostvdg/jenkins-job-builder,phinexus/jenkins-job-builder,onshape/jenkins-job-builder,onshape/jenkins-job-builder,dataxu/jenkins-job-builder,gforcada/jenkins-job-builder,tamac-io/jenkins-job-builder,twc-openstack/jenkins-job-builder,twc-openstack/jenkins-job-builder,OSSystems/jenkins-job-builder,onshape/jenkins-job-builder,dataxu/jenkins-job-builder,onshape/jenkins-job-builder,rsig/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,twc-openstack/jenkins-job-builder,joostvdg/jenkins-job-builder,lukas-bednar/jenkins-job-builder,gforcada/jenkins-job-builder,joostvdg/jenkins-job-builder,gforcada/jenkins-job-builder,joostvdg/jenkins-job-builder,puppetlabs/jenkins-job-builder,lukas-bednar/jenkins-job-builder,tamac-io/jenkins-job-builder,puppetlabs/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,wikimedia/integration-jenkins-job-builder,puppetlabs/jenkins-job-builder,puppetlabs/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,dataxu/jenkins-job-builder,dataxu/jenkins-job-builder,phinexus/jenkins-job-builder,wsoula/jenkins-job-builder,twc-openstack/jenkins-job-builder,OSSystems/jenkins-job-builder,rsig/jenkins-job-builder,tamac-io/jenkins-job-builder,wsoula/jenkins-job-builder