add docs (#4019)

Signed-off-by: Nok <nok.lam.chan@quantumblack.com>

docs/source/nodes_and_pipelines/nodes.md

@@ -74,6 +74,7 @@ A syntax describes function inputs and outputs. This syntax allows different Pyt
 | `None`                     | No input        | `def f()`                   | `f()`                                 |
 | `'a'`                      | Single input    | `def f(arg1)`               | `f(a)`                                |
 | `['a', 'b']`               | Multiple inputs | `def f(arg1, arg2)`         | `f(a, b)`                             |
+| `['a', 'b', 'c']`          | Variable inputs | `def f(arg1, *args)`.       | `f(arg1, arg2, arg3)`                 |
 | `dict(arg1='x', arg2='y')` | Keyword inputs  | `def f(arg1, arg2)`         | `f(arg1=x, arg2=y)`                   |
 
 ### Syntax for output variables
@@ -87,6 +88,9 @@ A syntax describes function inputs and outputs. This syntax allows different Pyt
 
 Any combinations of the above are possible, except nodes of the form `node(f, None, None)` (at least a single input or output must be provided).
 
+## `*args` node functions
+It is common to have functions that take an arbitrary number of inputs, like a function that combines multiple dataframes. You can use the `*args` argument in the node function, while simply declaring the names of the datasets in the node's inputs.
+
 ## `**kwargs`-only node functions
 
 Sometimes, when creating reporting nodes for instance, you need to know the names of the datasets that your node receives, but you might not have this information in advance. This can be solved by defining a `**kwargs`-only function: