JDK 15 RFR of JDK-8245146: Update description of SourceVersion.RELEASE_15 with text blocks

[Joe Darcy]

Hello,

As text blocks (JDK-8236934) have been added as a standard feature in 
JDK 15, the description of SourceVersion.RELEASE_15 should be updated 
accordingly. Please review the patch below which does this.

Thanks,

-Joe

diff -r 212f5084ac72 
src/java.compiler/share/classes/javax/lang/model/SourceVersion.java
--- 
a/src/java.compiler/share/classes/javax/lang/model/SourceVersion.java 
Sun May 17 11:09:52 2020 -0400
+++ 
b/src/java.compiler/share/classes/javax/lang/model/SourceVersion.java 
Sun May 17 10:15:20 2020 -0700
@@ -62,7 +62,7 @@
       *  13: no changes (switch expressions and text blocks in preview)
       *  14: switch expressions (pattern matching and records in
       *      preview, text blocks in preview again)
-     *  15: TBD
+     *  15: text blocks (records and pattern matching in preview again)
       */

      /**
@@ -211,6 +211,8 @@
       * The version recognized by the Java Platform, Standard Edition
       * 15.
       *
+     * Additions in this release include text blocks.
+     *
       * @since 15
       */
       RELEASE_15;