Merge pull request #4 from zeminzhou/zeminzhou/keyspace_rfc

doc: support TiCDC for keyspace

docs/design/2022-12-07-keyspace.md

@@ -70,6 +70,10 @@ If "keyspace-name" is set when executing the BR command, it supports restore and
 
 If "keyspace-name" is set when executing the Lightning command, it will encode key to add keyspace prefix before appends rows to the SST file, and decode key before records conflicts data into TiDB table.  
 
+#### TiCDC
+
+If "keyspace-name" is set when creating a changefeed to TiCDC server, the server only captures the table data within that keyspace for the changefeed. For changefeed configured with keyspace, the TiCDC server will encode the table range with the keyspace prefix to subscribe the table data to TiKV server, and it will also decode the keyspace prefix from the key that received from TiKV server to get the table data. If "keyspace-name" is not set, the TiCDC server will not capture the table data of [API V2](https://github.com/tikv/rfcs/blob/master/text/0069-api-v2.md#tikv-server).
+
 #### TiDB GC
 
 ##### Concepts of GC management type: