Expand key_hint doc

trillian.proto

@@ -222,7 +222,11 @@ message SignedLogRoot {
   // key_hint is a hint to identify the public key for signature verification.
   // key_hint is not authenticated and may be incorrect or missing, in which
   // case all known public keys may be used to verify the signature.
-  // key_hint MAY contain the LogID encoded as a big endian 64 bit integer.
+  // When directly communicating with a Trillian gRPC server, the key_hint will
+  // typically contain the LogID encoded as a big-endian 64-bit integer;
+  // however, in other contexts the key_hint is likely to have different
+  // contents (e.g. it could be a GUID, a URL + TreeID, or it could be
+  // derived from the public key itself).
   bytes key_hint = 7;
 }