Added documentation for trust-boundary-violation sink

Author: egregius313

docs/codeql/codeql-language-guides/customizing-library-models-for-java.rst

@@ -349,6 +349,7 @@ The following sink kinds are supported:
 - **response-splitting**: A sink that can be used for HTTP response splitting, such as in calls to **HttpServletResponse.setHeader**.
 - **sql-injection**: A sink that can be used for SQL injection, such as in a **Statement.executeQuery** call.
 - **template-injection**: A sink that can be used for server side template injection, such as in a **Velocity.evaluate** call.
+- **trust-boundary-violation**: A sink that can be used to cross a trust boundary, such as a server's HTTP Session.
 - **url-redirection**: A sink that can be used to redirect the user to a malicious URL, such as in a **Response.temporaryRedirect** call.
 - **xpath-injection**: A sink that can be used for XPath injection, such as in a **XPath.evaluate** call.
 - **xslt-injection**: A sink that can be used for XSLT injection, such as in a **Transformer.transform** call.