Improve description of path variables

Original commit by @finefoot, re-applied due to merge conflict.

docs/usage/general/environment.rst.inc

@@ -93,23 +93,32 @@ Some automatic "answerers" (if set, they automatically answer confirmation quest
     answer or ask you interactively, depending on whether retries are allowed (they by default are
     allowed). So please test your scripts interactively before making them a non-interactive script.
 
+.. _XDG env var: https://specifications.freedesktop.org/basedir-spec/0.6/ar01s03.html
+
 Directories and files:
     BORG_BASE_DIR
-        Defaults to '$HOME', '~$USER', '~' (in that order)'.
-        If we refer to ~ below, we in fact mean BORG_BASE_DIR.
+        Defaults to ``$HOME`` or ``~$USER`` or ``~`` (in that order).
+        If you want to move all borg-specific folders to a custom path at once, all you need to do is
+        to modify ``BORG_BASE_DIR``: the other paths for cache, config etc. will adapt accordingly
+        (assuming you didn't set them to a different custom value).
     BORG_CACHE_DIR
-        Defaults to '~/.cache/borg'. This directory contains the local cache and might need a lot
+        Defaults to ``$BORG_BASE_DIR/.cache/borg``. If ``BORG_BASE_DIR`` is not explicitly set while
+        `XDG env var`_ ``XDG_CACHE_HOME`` is set, then ``$XDG_CACHE_HOME/borg`` is being used instead.
+        This directory contains the local cache and might need a lot
         of space for dealing with big repositories. Make sure you're aware of the associated
         security aspects of the cache location: :ref:`cache_security`
     BORG_CONFIG_DIR
-        Defaults to '~/.config/borg'. This directory contains the whole config directories. See FAQ
-        for security advisory about the data in this directory: :ref:`home_config_borg`
+        Defaults to ``$BORG_BASE_DIR/.config/borg``. If ``BORG_BASE_DIR`` is not explicitly set while
+        `XDG env var`_ ``XDG_CONFIG_HOME`` is set, then ``$XDG_CONFIG_HOME/borg`` is being used instead.
+        This directory contains all borg configuration directories, see the FAQ
+        for a security advisory about the data in this directory: :ref:`home_config_borg`
     BORG_SECURITY_DIR
-        Defaults to '~/.config/borg/security'. This directory contains information borg uses to
-        track its usage of NONCES ("numbers used once" - usually in encryption context) and other
-        security relevant data. Will move with BORG_CONFIG_DIR variable unless specified.
+        Defaults to ``$BORG_CONFIG_DIR/security``.
+        This directory contains information borg uses to track its usage of NONCES ("numbers used
+        once" - usually in encryption context) and other security relevant data.
     BORG_KEYS_DIR
-        Defaults to '~/.config/borg/keys'. This directory contains keys for encrypted repositories.
+        Defaults to ``$BORG_CONFIG_DIR/keys``.
+        This directory contains keys for encrypted repositories.
     BORG_KEY_FILE
         When set, use the given filename as repository key file.
     TMPDIR