manpages: discuss about USK, SSK in fcpgenkey (1)

Add a discussion about the sameness of USK and SSK keys into
the manpage for fcpgenkey as this is one of the things I found
harder to understand as a newbie.

manpages/fcpgenkey.1

@@ -11,9 +11,29 @@ fcpgenkey \- generate a single Freenet SSK@/USK@ keypair
 fcpgenkey is a simple command\-line FCP client program for generating
 new Freenet SSK@ or USK@ public/private keypairs.
 
-The keypair generated is written to standard output stream, with
-the public key, then a newline, then the private key.
+The keypair generated is written to standard output stream as SSK@
+keys, with the public key, then a newline, then the private key.
 
+.SH "Equivalence between SSKs and USKs"
+.LP
+USK@ are really just a user-friendly wrapper around SSK@, which hide
+the process of searching for more recent versions of a site. Any
+address written using an USK@ key can also be written using an SSK@
+key.
+.TP
+.B
+Syntax of USK@ key:
+USK@<key>/<name_of_site>/<version#>/
+.TP
+.B
+Syntax of equivalent SSK@ key:
+SSK@<key>/<name_of_site>-<version#>/
+.TP
+.B
+These two keys are completely equivalent and can be inserted as either one:
+USK@z6lnO...ACAAE/freenetdocwiki/17/
+.br
+SSK@z6lnO...ACAAE/freenetdocwiki-17/
 .SH "OPTIONS"
 .LP 
 .TP