Adds HelpDesc, an optional, more detailed version of HelpText

Adds better support for multiple line HelpText by changing it into a string array.

Changed all TShock commands's HelpText to reflect this change. Instead of a simple HelpText = "string", it will now require HelpText = new[] {"string1", "string2", "stringx"} as you're obviously familiar with.

This will, however, break other developer's plugins which use HelpText.

I felt like this would be an useful change because line-break(\n) has it's own limitations based on Terraria's Chat Handling system. If more than 5 line breaks are made, the lines would overlap each other, since Terraria can only show 5 lines at a time.