[LIVY-398][DOC] Update rest API to reflect the changes of shared context session

LIVY-194 changes the semantics of session in Livy and add supports of shared context for Livy session, so here change the docs in REST API to reflect the changes of LIVY-194.

Author: jerryshao <sshao@hortonworks.com>

Closes #47 from jerryshao/LIVY-398.

docs/rest-api.md

@@ -78,7 +78,7 @@ Creates a new interactive Scala, Python, or R shell in the cluster.
   <tr><th>Name</th><th>Description</th><th>Type</th></tr>
   <tr>
     <td>kind</td>
-    <td>The session kind (required)</td>
+    <td>The session kind<sup><a href="#footnote1">[1]</a></sup></td>
     <td><a href="#session-kind">session kind</a></td>
   </tr>
   <tr>
@@ -153,6 +153,10 @@ Creates a new interactive Scala, Python, or R shell in the cluster.
   </tr>
 </table>
 
+<a id="footnote1">1</a>: Starting with version 0.5.0-incubating this field is not required. To be
+compatible with previous versions users can still specify this with spark, pyspark or sparkr,
+implying that the submitted code snippet is the corresponding kind.
+
 #### Response Body
 
 The created <a href="#session">Session</a>.
@@ -266,8 +270,17 @@ Runs a statement in a session.
     <td>The code to execute</td>
     <td>string</td>
   </tr>
+  <tr>
+    <td>kind</td>
+    <td>The kind of code to execute<sup><a href="#footnote2">[2]</a></sup></td>
+    <td><a href="#session-kind">code kind</a></td>
+  </tr>
 </table>
 
+<a id="footnote2">2</a>: If session kind is not specified or the submitted code is not the kind
+specified in session creation, this field should be filled with correct kind.
+Otherwise Livy will use kind specified in session creation as the default code kind.
+
 #### Response Body
 
 The <a href="#statement">statement</a> object.
@@ -610,11 +623,7 @@ A session represents an interactive shell.
   </tr>
   <tr>
     <td><a href="#pyspark">pyspark</a></td>
-    <td>Interactive Python 2 Spark session</td>
-  </tr>
-  <tr>
-    <td><a href="#pyspark3">pyspark3</a></td>
-    <td>Interactive Python 3 Spark session</td>
+    <td>Interactive Python Spark session</td>
   </tr>
   <tr>
     <td>sparkr</td>
@@ -622,22 +631,28 @@ A session represents an interactive shell.
   </tr>
 </table>
 
+Starting with version 0.5.0-incubating, each session can support all three Scala, Python and R
+interpreters. The ``kind`` field in session creation is no longer required, instead users should
+specify code kind (spark, pyspark or sparkr) during statement submission.
+
+To be compatible with previous versions, users can still specify ``kind`` in session creation,
+while ignoring ``kind`` in statement submission. Livy will then use this session
+``kind`` as default kind for all the submitted statements.
+
+If users want to submit code other than default ``kind`` specified in session creation, users
+need to specify code kind (spark, pyspark or sparkr) during statement submission.
+
 #### pyspark
+
 To change the Python executable the session uses, Livy reads the path from environment variable
 ``PYSPARK_PYTHON`` (Same as pyspark).
 
-Like pyspark, if Livy is running in ``local`` mode, just set the environment variable.
-If the session is running in ``yarn-cluster`` mode, please set
-``spark.yarn.appMasterEnv.PYSPARK_PYTHON`` in SparkConf so the environment variable is passed to
-the driver.
-
-#### pyspark3
-To change the Python executable the session uses, Livy reads the path from environment variable
-``PYSPARK3_PYTHON``.
+Starting with version 0.5.0-incubating, session kind "pyspark3" is removed, instead users require
+to set ``PYSPARK_PYTHON`` to python3 executable.
 
 Like pyspark, if Livy is running in ``local`` mode, just set the environment variable.
 If the session is running in ``yarn-cluster`` mode, please set
-``spark.yarn.appMasterEnv.PYSPARK3_PYTHON`` in SparkConf so the environment variable is passed to
+``spark.yarn.appMasterEnv.PYSPARK_PYTHON`` in SparkConf so the environment variable is passed to
 the driver.
 
 ### Statement