Add docstring to Powell (#2217)

Summary:
This was missing.

Pull Request resolved: #2217

Reviewed By: SebastianAment

Differential Revision: D54176182

Pulled By: Balandat

fbshipit-source-id: 5b2c59e960de90aaaaca61928a0f2474958e5f3e

Author: Balandat

botorch/test_functions/synthetic.py

@@ -586,6 +586,19 @@ def evaluate_true(self, X: Tensor) -> Tensor:
 
 
 class Powell(SyntheticTestFunction):
+    r"""Powell synthetic test function.
+
+    `d`-dim function (usually evaluated on hypercube `[-4, 5]^d`):
+
+        P(x) = sum_{i=1}^d/4 (
+            (x_{4i-3} + 10 x_{4i-2})**2
+            + 5 (x_{4i-1} - x_{4i})**2
+            + (x_{4i-2} - 2 x_{4i-1})**4
+            + 10 (x_{4i-3} - x_{4i})**4
+        )
+
+    P has a global minimizer at `z = (0, ..., 0)` with `P(z) = 0`.
+    """
 
     _optimal_value = 0.0