doc: Fix OpenPGP key update.

wiki/src/doc/about/openpgp_keys.mdwn

@@ -91,4 +91,5 @@ There are multiple ways to get this OpenPGP public key:
 - download it from this website: [[!tails_website tails-signing.key]]
 - fetch it from your favourite keyserver.
 
-FIXME: Mention that downloading the key again can fetch new or updated signatures.
+If you already have Tails signing key but download it again, it can update the
+list of existing signatures of the key.