Added documentation for BORG_CONFIG_DIR

(cherry picked from commit 764a62f42c41ef85fb916773e085f67a7c77eda6)

docs/usage_general.rst.inc

@@ -197,6 +197,8 @@ Directories and files:
     BORG_CACHE_DIR
         Default to '~/.cache/borg'. This directory contains the local cache and might need a lot
         of space for dealing with big repositories).
+    BORG_CONFIG_DIR
+        Default to '~/.config/borg'. This directory contains the whole config directories.
 
 Building:
     BORG_OPENSSL_PREFIX