Figure.makecpt: Improve the docstring of the 'reverse' parameter (#3009)

Co-authored-by: Michael Grund <[email protected]>

Author: yvonnefroehlich

pygmt/src/makecpt.py

@@ -121,10 +121,12 @@ def makecpt(**kwargs):
         level :func:`pygmt.makecpt` is called.
     reverse : str
         Set this to ``True`` or **c** [Default] to reverse the sense of color
-        progression in the master CPT. Set this to **z** to reverse the sign
-        of z-values in the color table. Note that this change of z-direction
-        happens before ``truncate`` and ``series`` values are used so the
-        latter must be compatible with the changed z-range. See also
+        progression in the master CPT. Also the foreground and background colors
+        are exchanged, including those specified by :gmt-term:`COLOR_BACKGROUND` and
+        :gmt-term:`COLOR_FOREGROUND`. Set this to **z** to reverse the sign of z-values
+        in the color table. Note that this change of the z-direction happens before
+        the ``truncate`` and ``series`` values are used, so the latter must be
+        compatible with the changed z-range. See also
         :gmt-docs:`reference/features.html#manipulating-cpts`.
     overrule_bg : str
         Overrule background, foreground, and NaN colors specified in the master