Walk and deep copy in Lua filters

[Bastien DUMONT <bastien.dumont-VwIFZPTo/vqsTnJN9+BGXg@public.gmane.org>]

Hi all,

I just discovered by chance that the method walk returns a deep copy of the passed object, leaving the original object untouched. See the following test case:

```deep-copy.lua
local orig = pandoc.Emph({ pandoc.Str('one'), pandoc.Space(), pandoc.Str('two') })
local copy = orig:walk({})
table.insert(copy.content, pandoc.Space())
table.insert(copy.content, pandoc.Str('three'))
assert(#orig.content == 3)
assert(#copy.content == 5)
```
`pandoc -L deep-copy.lua <<< ''` should return nothing (i.e. no error).

However, in the documentation, it is only stated that the “Result” is the “filtered block/inline element”: from this indication alone, it seems not obvious to me whether the passed object is changed (as it is customary with functions operating on Lua tables or userdata) or the method returns a new object.

To be clear, I am very happy with the current behaviour, for it gives a very simple means to get several deep copies of an objet and to manipulate them separately without having to regenerate the original object (e.g. via rather expensive calls to `pandoc.read()`). Nevertheless, since it is not really documented, can it be considered to be deliberate and stable? If this is the case, could it be possible to make explicit in the documentation, under the header “Result”, that the original block or inline element is left unchanged?

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