docs: examples: improve domain_seed comments

docs/multidomain-site-example/domains/alpha_centauri.conf

@@ -8,6 +8,7 @@
   },
 
   -- 32 byte random data in hexadecimal encoding
+  -- This data must be unique among all sites and domains!
   -- Can be generated using: echo $(hexdump -v -n 32 -e '1/1 "%02x"' </dev/urandom)
   domain_seed = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx',
 

docs/site-example/site.conf

@@ -16,7 +16,7 @@
   site_code = 'ffxx',
 
   -- 32 bytes of random data, encoded in hexadecimal
-  -- Must be the same for all nodes in one mesh domain
+  -- This data must be unique among all sites and domains!
   -- Can be generated using: echo $(hexdump -v -n 32 -e '1/1 "%02x"' </dev/urandom)
   domain_seed = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx',