Update db cache table documentation

doc/database/db_cache.md

@@ -1,11 +1,13 @@
 Table cache
 ===========
 
+Stores temporary data
+
 | Field        | Description                        | Type         | Null | Key | Default             | Extra |
 | ------------ | ---------------------------------- | ------------ | ---- | --- | ------------------- | ----- |
-| k            | horizontal width + url or resource | varchar(255) | NO   | PRI | NULL                |       |
-| v            | OEmbed response from site          | text         | NO   |     | NULL                |       |
+| k            | cache key                          | varchar(255) | NO   | PRI | NULL                |       |
+| v            | cached serialized value            | text         | NO   |     | NULL                |       |
+| expires      | datetime of cache expiration       | datetime     | NO   | MUL | 0001-01-01 00:00:00 |       |
 | updated      | datetime of cache insertion        | datetime     | NO   | MUL | 0001-01-01 00:00:00 |       |
-| expire_mode  |                                    | int(11)      | NO   |     | 0                   |       |
 
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