parsers: skip_empty_values is not documented

[Youssef Rizkalla]

I've noticed that skip_empty_values is not mentioned in the documentation as a possible configuration parameter. There are situations in which a multiline rule expects an empty line, such as in golang stack traces and this option makes it possible to use a regex parser that will correctly handle such a case for structured inputs.

Looking at the implementation, it seems like supporting this option through the public API was intentional. If this is the case, I think that it would be beneficial to add it to the public docs as well.