DOC: added plotting module to the api reference docs (pandas-dev#19780)

Author: mehemken

doc/source/api.rst

@@ -2388,15 +2388,23 @@ Style Export and Import
    Styler.to_excel
 
 Plotting
-~~~~~~~~
+--------
 
-.. currentmodule:: pandas
+.. currentmodule:: pandas.plotting
+
+The following functions are contained in the `pandas.plotting` module.
 
 .. autosummary::
    :toctree: generated/
 
-   plotting.register_matplotlib_converters
-   plotting.deregister_matplotlib_converters
+   andrews_curves
+   bootstrap_plot
+   deregister_matplotlib_converters
+   lag_plot
+   parallel_coordinates
+   radviz
+   register_matplotlib_converters
+   scatter_matrix
 
 .. currentmodule:: pandas