[DOCS] Advise user to shutdown pretender servers

[dustinfarris]

Updating old documentation. I'm not sure why the original docs advised running the initializer method; the startMirage function should work just fine—no helper file should be necessary.

Closes #915 (not automatically, though)

[robneville73]

Yes this seems to fix the issues with having multiple pretenders running

[samselikoff]

The initializer method code was taken from a @rwjblue gist... Robert, is it ok to just use

import { startMirage } from 'yourapp/initializers/ember-cli-mirage'

and call it without passing in the container?

[rwjblue]

Sure, if the mirage initializer does not need the container, then we definitely don't need to pass it in.