Lua filters: AttributeList constructor (and constructor docs in general)

[wlupton]

Note: This has been spawned from #7657, to which @tarleb commented:

I plan to generate the constructor documentation from the Haskell code, and to make the docs accessible from within the Lua interpreter.

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The 2.15 release notes mention AttributeList() but I didn't see any info on how to use it.

More generally, I feel that the lua constructor documentation is rather sparse. Compare pandoc.Image, which states the data type of each field, with pandoc.Image(), which is quite vague. For example, I think that caption can in fact be either a string or a list of Inlines?