(maint) Clarify docs and add new tests

Based on a Stack Overflow question, it was noted that the documentation
for `file_line` isn't complete and the underlying behaviour somewhat
confusing.

https://stackoverflow.com/questions/46468922/how-to-change-the-contents-of-a-file-by-using-puppet/46473458

In this patch I add additional unit tests and better examples and
complete documentation.

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