Add a rough explanation of --extreme to output of --help.

src/xz/message.c

@@ -1075,6 +1075,10 @@ message_help(bool long_help)
 "  -0 .. -9            compression preset; 0-2 fast compression, 3-5 good\n"
 "                      compression, 6-9 excellent compression; default is 6"));
 
+	puts(_(
+"  -e, --extreme       use more CPU time when encoding to increase compression\n"
+"                      ratio without increasing memory usage of the decoder"));
+
 	puts(_(
 "  -M, --memory=NUM    use roughly NUM bytes of memory at maximum; 0 indicates\n"
 "                      the default setting, which depends on the operation mode\n"