[tools] Print unicode links on gen-* tools output

As Behdad's review

src/gen-arabic-table.py

diff --git a/src/gen-arabic-table.py b/src/gen-arabic-table.py
index 5e934a4..7fe26b0 100755
--- a/src/gen-arabic-table.py
+++ b/src/gen-arabic-table.py
@@ -1,16 +1,17 @@
 #!/usr/bin/env python
 
-# Input files:
-# - https://unicode.org/Public/12.0.0/ucd/ArabicShaping.txt
-# - https://unicode.org/Public/12.0.0/ucd/UnicodeData.txt
-# - https://unicode.org/Public/12.0.0/ucd/Blocks.txt
-
 from __future__ import print_function, division, absolute_import
 
 import io, os.path, sys
 
 if len (sys.argv) != 4:
-	print ("usage: ./gen-arabic-table.py ArabicShaping.txt UnicodeData.txt Blocks.txt", file=sys.stderr)
+	print ("""usage: ./gen-arabic-table.py ArabicShaping.txt UnicodeData.txt Blocks.txt
+
+Input files, as of Unicode 12:
+* https://unicode.org/Public/UCD/latest/ucd/ArabicShaping.txt
+* https://unicode.org/Public/UCD/latest/ucd/UnicodeData.txt
+* https://unicode.org/Public/UCD/latest/ucd/Blocks.txt
+""", file=sys.stderr)
 	sys.exit (1)
 
 files = [io.open (x, encoding='utf-8') for x in sys.argv[1:]]