Merge pull request matplotlib#4039 from Pybonacci/fix-stylesheets-docs

DOC : better reflect where config folder is in style docs

added some fixes in order to use the result obtained from `mpl._get_configdir()`

doc/users/style_sheets.rst

@@ -25,15 +25,15 @@ Defining your own style
 =======================
 
 You can create custom styles and use them by calling ``style.use`` with the
-path or URL to the style sheet. Alternatively, if you add your
-``<style-name>.mplstyle`` file to ``~/.matplotlib/stylelib`` (you may need to
-create this directory), you can reuse your custom style sheet with a call to
-``style.use(<style-name>)``. Note that a custom style sheet in
-``~/.matplotlib/stylelib`` will override a style sheet defined by matplotlib if
-the styles have the same name.
+path or URL to the style sheet. Alternatively, if you add your ``<style-name>.mplstyle`` 
+file to ``mpl_configdir/stylelib`, you can reuse your custom style sheet with a call to 
+``style.use(<style-name>)``. By default ``mpl_configdir`` should be ``~/.config/matplotlib``, 
+but you can check where yours is with ``matplotlib.get_configdir()``, you may need to 
+create this directory. Note that a custom style sheet in ``mpl_configdir/stylelib`` 
+will override a style sheet defined by matplotlib if the styles have the same name.
 
 For example, you might want to create
-``~/.matplotlib/stylelib/presentation.mplstyle`` with the following::
+``mpl_configdir/stylelib/presentation.mplstyle`` with the following::
 
    axes.titlesize : 24
    axes.labelsize : 20