Request change to documentation to reflect usage requirement

Issue Summary

SpamSpan filter will only work properly if the mailto URL has no parameters.

For example, this will work:

<a href="mailto:name@example.com">name@example.com</a>

This will fail in a very ugly way:

<a href="mailto:name@example.com?subject=foo&cc=bar@example.com">name@example.com</a>

I can understand that it might be a bear to get the regex to work for the more complex URL, but it would be great if you could document the behavior so people don't have to install the filter and find out it doesn't work when users start complaining.