Add javadoc about ThreadContextPermission for stashWithOrigin and sta…

…shAndMergeHeaders (opensearch-project#15051)

Signed-off-by: Craig Perkins <cwperx@amazon.com>

server/src/main/java/org/opensearch/common/util/concurrent/ThreadContext.java

@@ -213,6 +213,13 @@ public Writeable captureAsWriteable() {
      * For example, a user might not have permission to GET from the tasks index
      * but the tasks API will perform a get on their behalf using this method
      * if it can't find the task in memory.
+     *
+     * Usage of stashWithOrigin is guarded by a ThreadContextPermission. In order to use
+     * stashWithOrigin, the codebase needs to explicitly be granted permission in the JSM policy file.
+     *
+     * Add an entry in the grant portion of the policy file like this:
+     *
+     * permission org.opensearch.secure_sm.ThreadContextPermission "stashWithOrigin";
      */
     public StoredContext stashWithOrigin(String origin) {
         SecurityManager sm = System.getSecurityManager();
@@ -228,6 +235,13 @@ public StoredContext stashWithOrigin(String origin) {
      * Removes the current context and resets a new context that contains a merge of the current headers and the given headers.
      * The removed context can be restored when closing the returned {@link StoredContext}. The merge strategy is that headers
      * that are already existing are preserved unless they are defaults.
+     *
+     * Usage of stashAndMergeHeaders is guarded by a ThreadContextPermission. In order to use
+     * stashAndMergeHeaders, the codebase needs to explicitly be granted permission in the JSM policy file.
+     *
+     * Add an entry in the grant portion of the policy file like this:
+     *
+     * permission org.opensearch.secure_sm.ThreadContextPermission "stashAndMergeHeaders";
      */
     public StoredContext stashAndMergeHeaders(Map<String, String> headers) {
         SecurityManager sm = System.getSecurityManager();