CLI help: never a password used elsewhere as the words of a key

Spec §38.1: the official SDK SHOULD warn not to reuse a password, since
once the date has come the capsule lets whoever holds it test guesses.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/datekeys/main.go

@@ -81,7 +81,9 @@ was published (spec v0.15, §63 step 9.c).
 least 6 different words of 3 or more letters that open it with decrypt,
 instead of a .dkk
 (wordkey). Case, accents and extra spaces do not matter. -words leaves them
-in the shell history; -words-file reads them from a file.
+in the shell history; -words-file reads them from a file. Never use a
+password used anywhere else: once the date has come, the capsule lets
+whoever holds it test guesses of it.
 
 -new-words FILE draws the words at random instead, and writes them to the new
 file FILE: -word-count words, 7 by default, of the list -dic of 7776 words,