Update API-related documentation in Versioning Policy

This PR updates API-related documentation in Versioning Policy. It is an API when it is documented - we intentionally do not document some.

Author: Hyukjin Kwon <[email protected]>

Closes #597 from HyukjinKwon/update-api-policy1.

Author: HyukjinKwon

site/versioning-policy.html

@@ -187,11 +187,8 @@ <h3>Alpha components</h3>
 
 <h3>API compatibility</h3>
 
-<p>An API is any public class or interface exposed in Spark that is not marked as &#8220;developer API&#8221; or 
-&#8220;experimental&#8221;. Release A is API compatible with release B if code compiled against release A 
-<em>compiles cleanly</em> against B. Currently, does not guarantee that a compiled application that is 
-linked against version A will link cleanly against version B without re-compiling. Link-level 
-compatibility is something we&#8217;ll try to guarantee in future releases.</p>
+<p>In general, An API is any public class or interface documented in Spark, e.g., <a href="https://spark.apache.org/docs/latest/api/scala/org/apache/spark/index.html">ScalaDoc</a>.
+We try to guarantee both source compatibility and binary compatibility between releases.</p>
 
 <p>Note, however, that even for features &#8220;developer API&#8221; and &#8220;experimental&#8221;, we strive to maintain 
 maximum compatibility. Code should not be merged into the project as &#8220;experimental&#8221; if there is 

versioning-policy.md

@@ -39,11 +39,8 @@ try to. Once they are marked "stable" they have to follow these guidelines.
 
 <h3>API compatibility</h3>
 
-An API is any public class or interface exposed in Spark that is not marked as "developer API" or 
-"experimental". Release A is API compatible with release B if code compiled against release A 
-_compiles cleanly_ against B. Currently, does not guarantee that a compiled application that is 
-linked against version A will link cleanly against version B without re-compiling. Link-level 
-compatibility is something we'll try to guarantee in future releases. 
+In general, An API is any public class or interface documented in Spark, e.g., <a href="https://spark.apache.org/docs/latest/api/scala/org/apache/spark/index.html">ScalaDoc</a>.
+We try to guarantee both source compatibility and binary compatibility between releases.
 
 Note, however, that even for features "developer API" and "experimental", we strive to maintain 
 maximum compatibility. Code should not be merged into the project as "experimental" if there is