Add header description to e2doc

[Denneisk]

Very minor change to e2doc and e2descriptions that makes it so you can have more text than just function descriptions in the generated documentation. An example output is provided in the wiki.

Part of my plans to improve the documentation.

I was unsure how to store the description so I figured reserving identifiers starting with _doc_ surely couldn't be the worst way.
This can, of course, be changed if it causes problems.

[vurvdev]

I don't think it should be stored like this, E2Helper.Descriptions is meant to be used for functions, and seen in game.

I think the better way to go about reworking the documentation is this way:

1. Delete these autogenerated wiki pages. They just bloat actual handwritten pages.
2. Make a website similar to StarfallEx's to display documentation better: I already made a prototype in svelte a while back.
3. Rework documentation of e2functions to be from e2function attributes

e.g.

[doc = "This function does xyz"]
e2function test()

end

Also, extensions already have descriptions, given by E2Lib.registerExtension. You could make datasignals an extension (enabled by default) and have the description in there, and then use these descriptions for the doc generator for now.