docs: add utility for functions.py

Author: yucongalicechen

doc/utilities/functions_utility.rst

@@ -2,3 +2,22 @@
 
 Functions Utility
 =================
+
+The `diffpy.labpdfproc.functions` module provides tools
+for computing and applying absorption correction (cve) to 1D diffraction patterns.
+
+- `Gridded_circle`: This class supports absorption correction by
+  creating a uniform grid of points within a circle for a given radius and mu (linear absorption coefficient),
+  and computing the path length and effective volume for each grid point at a given angle.
+
+- `compute_cve`: This function computes the absorption correction curve for a given mu*D
+  (absorption coefficient mu and capillary diameter D).
+  For brute force computation, it averages the effective volume across all grid points,
+  then computes the cve values as the reciprocal of this average.
+  For polynomial interpolation, it uses pre-computed coefficients to quickly interpolate cve values for a given mu*D.
+  Polynomial interpolation is available for mu*D values between 0.5-6.
+
+- `apply_corr`: This function applies the computed absorption correction to the input diffraction pattern
+  by multiplying it with the computed cve, resulting in a corrected diffraction pattern.
+
+For a more in-depth tutorial for how to use these tools, click :ref:`here <Functions Example>`.