Fix the review findings in --help and config.example.yml

snapshot list says why a remote-only snapshot is listed by its remote
key. snapshot verify says what access to the destination it needs. The
note on the age-keygen file is two sentences in snapshot verify and
snapshot restore. remote nuke says "backup destination store".

config.example.yml names the commands that run without a snapshot.

Only help strings and comments change.

Model: fable-5-1

configs/config.example.yml

@@ -24,7 +24,8 @@ age_recipients:
 
 # Named snapshots - each snapshot can contain multiple paths
 # Each snapshot gets its own ID and can have snapshot-specific excludes
-# Required: every command needs at least one snapshot with at least one path
+# Required: at least one snapshot with at least one path. Only the config
+# commands, version and completion run without one
 snapshots:
   testing:
     paths: