docs(data/int/range): add module docstring (#7971)

src/data/int/range.lean

@@ -6,11 +6,22 @@ Authors: Mario Carneiro, Kenny Lau
 import data.int.basic
 import data.list.range
 
+/-!
+# Intervals in ℤ
+
+This file defines integer ranges. `range m n` is the set of integers greater than `m` and strictly
+less than `n`.
+
+## Note
+
+This could be unified with `data.list.intervals`. See the TODOs there.
+-/
+
 namespace int
 
 local attribute [semireducible] int.nonneg
 
-/-- List enumerating `[m, n)`. -/
+/-- List enumerating `[m, n)`. This is the ℤ variant of `list.Ico`. -/
 def range (m n : ℤ) : list ℤ :=
 (list.range (to_nat (n-m))).map $ λ r, m+r