Add doc comments

Packages/PFFFT/Tests/PFFFTTests/PFFFTTests.swift

@@ -2,5 +2,14 @@ import Testing
 @testable import PFFFT
 
 @Test func example() async throws {
+    let cache = PFFFT.SetupCache<Double>()
+
+    let ff = try cache.get(for: 1024, type: .real)
+
+    let input = Buffer<Double>(capacity: 1023)
+    let output = Buffer<Double>(capacity: 1024)
+
+    ff.fft(input: input, output: output, work: nil, sign: .forward)
+
     // Write your test here and use APIs like `#expect(...)` to check expected conditions.
 }