[apache#1124] docs(tez): Add the document of config option `tez.rss.c…

…lient.send.thread.num` (apache#1142)

### What changes were proposed in this pull request?

Add the document of config option `tez.rss.client.send.thread.num `

### Why are the changes needed?

Fix: apache#1124 

### Does this PR introduce _any_ user-facing change?

No.

docs/client_guide.md

@@ -243,6 +243,7 @@ Notice: this feature requires the MEMORY_LOCAL_HADOOP mode.
 | tez.rss.avoid.recompute.succeeded.task | false   | Whether to avoid recompute succeeded task when node is unhealthy or black-listed |
 | tez.rss.client.max.buffer.size | 3k | The max buffer size in map side. Control the size of each segment(WrappedBuffer) in the buffer. |
 | tez.rss.client.batch.trigger.num | 50 | The max batch of buffers to send data in map side. Affect the number of blocks sent to the server in each batch, and may affect rss_worker_used_buffer_size |
+| tez.rss.client.send.thread.num | 5 | The thread pool size for the client to send data to the server. |
 
 ### Netty Setting
 | Property Name                                       | Default | Description                                                                                                                                                                                                                                                                                                                         |