[SPARK-14882][DOCS] Clarify that Spark can be cross-built for other Scala versions

## What changes were proposed in this pull request?

Add simple clarification that Spark can be cross-built for other Scala versions.

## How was this patch tested?

Automated doc build

Author: Sean Owen <sowen@cloudera.com>

Closes #12757 from srowen/SPARK-14882.

docs/programming-guide.md

@@ -24,7 +24,8 @@ along with if you launch Spark's interactive shell -- either `bin/spark-shell` f
 
 <div data-lang="scala"  markdown="1">
 
-Spark {{site.SPARK_VERSION}} uses Scala {{site.SCALA_BINARY_VERSION}}. To write
+Spark {{site.SPARK_VERSION}} is built and distributed to work with Scala {{site.SCALA_BINARY_VERSION}} 
+by default. (Spark can be built to work with other versions of Scala, too.) To write
 applications in Scala, you will need to use a compatible Scala version (e.g. {{site.SCALA_BINARY_VERSION}}.X).
 
 To write a Spark application, you need to add a Maven dependency on Spark. Spark is available through Maven Central at: