Set appropriate params for man_pages generation.

`sphinx` can generate not only HTML docs but also man pages. Let's see
how far we can go to replace the more or less manually crafted man page
we provide currently.

docs/conf.py

@@ -254,8 +254,8 @@ latex_documents = [
 # One entry per manual page. List of tuples
 # (source start file, name, description, authors, manual section).
 man_pages = [
-    ('index', 'diceware', 'diceware Documentation',
-     ['Uli Fouquet'], 1)
+    ('index', 'diceware', 'create a passphrase',
+     ['Written by Uli Fouquet and contributors.'], 1)
 ]
 
 # If true, show URL addresses after external links.