add a bit of docs to the example

#include <qcor_qft>

// Here, we are saying this is what our QPE algorithm 

// expects as a kernel signature for user-provided 

// oracles. All oracles in this example must take a qreg and and int.

using QPEOracleSignature = KernelSignature<qreg, int>;

// Define our QPE algorithm, taking as input the 

// qreg to run on and an Oracle provided as a 

// qcor quantum kernel of the required function signature.

__qpu__ void QuantumPhaseEstimation(qreg q, QPEOracleSignature oracle) {

  // We have nQubits, the last one we use

  // as the state qubit, the others we use as the counting qubits

int main(int argc, char **argv) {

  auto q = qalloc(4);

  // Just pass the oracle kernel to the 

  // phase estimation algorithm.

  QuantumPhaseEstimation(q, compositeOp);

  q.print();

}