Clearify behavior of StringBuilderWriter constructor for null inputs

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/io/trunk@1681000 13f79535-47bb-0310-9956-ffa450edef68

Author: britter

src/main/java/org/apache/commons/io/output/StringBuilderWriter.java

@@ -54,8 +54,10 @@ public StringBuilderWriter(final int capacity) {
 
     /**
      * Construct a new instance with the specified {@link StringBuilder}.
+     * 
+     * <p>If {@code builder} is null a new instance with default capacity will be created.</p>
      *
-     * @param builder The String builder
+     * @param builder The String builder. May be null.
      */
     public StringBuilderWriter(final StringBuilder builder) {
         this.builder = builder != null ? builder : new StringBuilder();