Add procfn example

src/python_spectrometer/daq/zurich_instruments.py

@@ -39,7 +39,15 @@ Compare their results::
     spect_scope.take(n_pts=2**14, fs=14.6e3)
 
 The DAQ spectrum should show a peak at :math:`f=-500\,\mathrm{Hz}`,
-corresponding to the oscillator frequency.
+corresponding to the oscillator frequency. This is the shifted 0 Hz
+peak of the instrument's $1/f$ noise.
+
+Use ``procfn`` to compute the phase noise spectrum using the DAQ module::
+
+    spect_daq.procfn = lambda x, **_: np.angle(x)
+    spect_daq.processed_unit = 'rad'
+    spect_daq.drop('all')
+    spect_daq.take()
 
 """
 from __future__ import annotations