doc(utils): add spacing effect

+af_tools/+utils/spacedvector.m

 function vect = spacedvector(spacing, nPoints)
     % SPACEDVECTOR Returns a vector based on chosen spacing and number of points
+    %
+    % Spacing:
+    %   - linear     : points spaced linearily
+    %   - halfcosine : more dense at beginning and end of vector
+    %   - cosine     : more dense at the beginning and more spaced out at the end
+    %   - sine       : more dense at the end and more spaced out at the beginning
+    %
     % -----
     % (c) Copyright 2022-2023 University of Liege
     % Author: Thomas Lambert <t.lambert@uliege.be>