Documentation does not specify mcrypt requirement

Issue Summary

The documentation does mention the mcrypt module (now that I look at it in more depth) but there is no requirements section listing the mcrypt php extension. However it doesn't just not provide crypted passwords if you don't have it, it doesn't work. This requirement needs to be spelled out and optimally the module would check to see if mcrypt is present, and refuse to enable itself if not. But still, just spelling out the requirement in the documentation is probably enough.