doc: vlc.1: add VLC_USERDATA_PATH to the manpages

Like VLC_DATA_PATH AND VLC_LIB_PATH, but allows to override the path
matching with ~/.local/share on Linux or ~/Library/Application\ Support
on Darwin.

doc/vlc.1

@@ -98,6 +98,10 @@ loaded libraries and architecture-dependant executables (e.g. /usr/lib/).
 .B VLC_PLUGIN_PATH
 An extra directory to load VLC plugins from.
 
+.TP
+.B VLC_USERDATA_PATH
+The directory where the userdata will be stored (e.g. ~/.local/share/).
+
 .TP
 .B VLC_VERBOSE
 The level of verbosity for log messages