docs: explain XDG base dir and parameter expansion

bastimeyer · gravyboat · commit 177f878d6174 · 2022-11-29T10:38:00.000-08:00
The docs should be as explicit as possible, but this might confuse
inexperienced users, so an explanation might be useful.
diff --git a/docs/cli/config.rst b/docs/cli/config.rst
@@ -40,6 +40,17 @@ You can also specify the location yourself using the :option:`--config` option.
   Streamlink's Windows installers automatically create a config file if it doesn't exist yet, but on any
   other platform or installation method, you must create the file yourself.
 
+.. note::
+
+   The ``XDG_CONFIG_HOME`` environment variable is part of the `XDG base directory specification`_ (`Arch Wiki <xdg-base-dir-arch-wiki_>`_).
+
+   The ``${VARIABLENAME:-DEFAULTVALUE}`` syntax is explained `here <Parameter expansion_>`_.
+
+.. _XDG base directory specification: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
+.. _xdg-base-dir-arch-wiki: https://wiki.archlinux.org/title/XDG_Base_Directory
+.. _Parameter expansion: https://wiki.bash-hackers.org/syntax/pe
+
+
 Syntax
 ------
 
