(#3553) Explain that cron resources require time attributes

The cron resource docs previously read, "All fields except the command and the
user are optional, although specifying no periodic fields would result in the
command being executed every minute." This was factually incorrect; instead,
specifying no periodic fields results in a failure and an unhelpful error on
Puppet 2.6 and 2.7.

Although the issue will remain open as a behavior bug, this commit corrects
the documentation of which attributes are required. It also changes the @doc
string to a heredoc to simplify quote escaping.