Updated description of default session location.

- Include alternate location
- Include reference to `${XDB_DATA_HOME}` env variable

Author: georgeanderson

docs/restoring_previously_saved_environment.md

@@ -1,7 +1,8 @@
 # Restoring previously saved environment
 
 None of the previous saves are deleted (unless you explicitly do that). All save
-files are kept in `~/.tmux/resurrect/` directory.<br/>
+files are kept in `~/.tmux/resurrect/` directory, or `~/.local/share/tmux/resurrect`
+(unless `${XDG_DATA_HOME}` says otherwise).<br/>
 Here are the steps to restore to a previous point in time:
 
 - make sure you start this with a "fresh" tmux instance