doc(data/set/basic): add pointer to iff versions (#17300)

src/data/set/basic.lean

@@ -298,6 +298,7 @@ lemma nonempty_of_mem {x} (h : x ∈ s) : s.nonempty := ⟨x, h⟩
 theorem nonempty.not_subset_empty : s.nonempty → ¬(s ⊆ ∅)
 | ⟨x, hx⟩ hs := hs hx
 
+/-- See also `set.ne_empty_iff_nonempty` and `set.not_nonempty_iff_eq_empty`. -/
 theorem nonempty.ne_empty : ∀ {s : set α}, s.nonempty → s ≠ ∅
 | _ ⟨x, hx⟩ rfl := hx