models: Add OnCall.description for position clarification

My conversation with Jordan Schatz from 2013-11-12:

  Jordan:
    Can we add more descriptive text to the page it opens [the list
    page]?

  Trevor:
    Sure thing, just tell me what to drop in the template.

  Jordan:
    ...  A paragraph that helps users distinguish between network
    issues which go to Steven and beehive issues that go to us.