ZOOKEEPER-3373: need change description for "Single System Image" guarantee in document

In website, "Single System Image" is "A client will see the same view of the service regardless of the server that it connects to."

I want to change it to "Once connected, a client will see the same view of the service even if it switchs to another server"

Because the old one is a little misleading, if cluster has a outdated follower and a normal follower, I do not think a client will see the same view of the service regardless of the server that it connects to at its first connection.

Author: 数组越界 <ihavenoemail@163.com>

Reviewers: maoling <maoling199210191@sina.com>, Andor Molnár <andor@apache.org>, Michael Han <hanm@apache.org>

Closes #931 from kabike/ZOOKEEPER-3373

zookeeper-docs/src/main/resources/markdown/zookeeperOver.md

@@ -159,7 +159,9 @@ synchronization, it provides a set of guarantees. These are:
 * Atomicity - Updates either succeed or fail. No partial
   results.
 * Single System Image - A client will see the same view of the
-  service regardless of the server that it connects to.
+  service regardless of the server that it connects to. i.e., a
+  client will never see an older view of the system even if the
+  client fails over to a different server with the same session.
 * Reliability - Once an update has been applied, it will persist
   from that time forward until a client overwrites the update.
 * Timeliness - The clients view of the system is guaranteed to

zookeeper-docs/src/main/resources/markdown/zookeeperProgrammers.md

@@ -1054,7 +1054,9 @@ guarantees:
 
 * *Single System Image* :
     A client will see the same view of the service regardless of
-    the server that it connects to.
+    the server that it connects to. i.e., a client will never see an
+    older view of the system even if the client fails over to a
+    different server with the same session.
 
 * *Reliability* :
     Once an update has been applied, it will persist from that