Add docs

docs/checks.md

@@ -2055,7 +2055,7 @@ cmd = shlex.join(args)
 
 Categories: `itertools` `performance` `readability`
 
-When flattening an list of lists, use the `chain.from_iterable` function
+When flattening a list of lists, use the `chain.from_iterable()` function
 from the `itertools` stdlib package. This function is faster than native
 list/generator comprehensions or using `sum()` with a list default.
 
@@ -2083,9 +2083,13 @@ from itertools import chain
 
 rows = [[1, 2], [3, 4]]
 
-flat = chain.from_iterable(*rows)
+flat = chain.from_iterable(rows)
 ```
 
+Note: `chain.from_iterable()` returns an iterator, which means you might
+need to wrap it in `list()` depending on your use case. Refurb cannot
+detect this (yet), so this is something you will need to keep in mind.
+
 Note: `chain(*x)` may be marginally faster/slower depending on the length
 of `x`. Since `*` might potentially expand to a lot of arguments, it is
 better to use `chain.from_iterable()` when you are unsure.