Tweak wording, and comment on the resolution of the timestamp

src/github.com/matrix-org/dendrite/common/keydb/keydb.go

@@ -20,7 +20,7 @@ import (
 )
 
 // A Database implements gomatrixserverlib.KeyDatabase and is used to store
-// the public keys downloaded for other matrix servers.
+// the public keys for other matrix servers.
 type Database struct {
 	statements serverKeyStatements
 }

src/github.com/matrix-org/dendrite/common/keydb/server_key_table.go

@@ -31,7 +31,7 @@ CREATE TABLE IF NOT EXISTS server_keys (
 	-- Combined server name and key ID separated by the ASCII unit separator
 	-- to make it easier to run bulk queries.
 	server_name_and_key_id TEXT NOT NULL,
-	-- When the keys are valid until.
+	-- When the keys are valid until as a millisecond timestamp.
 	valid_until_ts BIGINT NOT NULL,
 	-- The raw JSON for the server key.
 	server_key_json TEXT NOT NULL,