LANG-1274: StrSubstitutor should state its thread safety (closes #207)

Add paragraph to class javadoc stating that StrSubstitutor is not thread safe.

src/changes/changes.xml

@@ -56,6 +56,7 @@ The <action> type attribute can be add,update,fix,remove.
     <action issue="LANG-1155" type="fix" dev="pschumacher" due-to="Saif Asif, Thiago Andrade">Add StringUtils#unwrap</action>
     <action issue="LANG-1160" type="add" dev="kinow">StringUtils#abbreviate should support 'custom ellipses' parameter</action>
     <action issue="LANG-1270" type="add" dev="pschumacher" due-to="Pierre Templier">Add StringUtils#isAnyNotEmpty and #isAnyNotBlank</action>
+    <action issue="LANG-1274" type="update" dev="pschumacher">StrSubstitutor should state its thread safety</action>
     <action issue="LANG-1277" type="update" dev="pschumacher" due-to="yufcuy">StringUtils#getLevenshteinDistance reduce memory consumption</action>
     <action issue="LANG-1279" type="update" dev="ggregory">Update Java requirement from Java 6 to 7.</action>
     <action issue="LANG-1143" type="update" dev="pschumacher" due-to="sebb">StringUtils should use toXxxxCase(int) rather than toXxxxCase(char)</action>

src/main/java/org/apache/commons/lang3/text/StrSubstitutor.java

@@ -118,6 +118,7 @@
  * names, but it has to be enabled explicitly by setting the
  * {@link #setEnableSubstitutionInVariables(boolean) enableSubstitutionInVariables}
  * property to <b>true</b>.
+ * <p>This class is <b>not</b> thread safe.</p>
  *
  * @since 2.2
  */