[Java] Note use of %ignore with %native

This is needed if you're wanting to effectively replace the generated
JNI wrapper for a C/C++ function.

Fixes https://sourceforge.net/p/swig/bugs/368/

Author: ojwb

Doc/Manual/Java.html

@@ -9062,6 +9062,11 @@ <H3><a name="Java_using_own_jni_functions">27.12.3 Using your own JNI functions<
 Here the default typemaps work for <tt>int</tt> and <tt>char *</tt>.
 </p>
 
+<p>
+Note that if you're wanting to effectively <b>replace</b> the JNI code generated for a C/C++ function then you'll need to use <tt>%ignore</tt> as well
+to tell SWIG not to automatically generate a JNI wrapper for it.
+</p>
+
 <p>
 In summary the <tt>%native</tt> directive is telling SWIG to generate the Java code to access the JNI C code, but not the JNI C function itself.
 This directive is only really useful if you want to mix your own hand crafted JNI code and the SWIG generated code into one Java class or package.