jni_generator: Document @CalledByNatives("InnerClassName").

Also added a note about a known issue that can cause build failures.

Bug: 967618
Change-Id: Iebf4df4a15453e525653ee2052f61e84c0e169f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632299
Auto-Submit: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663748}

base/android/jni_generator/README.md

@@ -195,6 +195,8 @@ If a native method is called without setting a mock in a unit test, an
 ### Calling Native -> Java
 
  * Methods annotated with `@CalledByNative` will have stubs generated for them.
+   * Inner class methods must provide the inner class name explicitly
+     (ex. `@CalledByNative("InnerClassName")`)
  * Just call the generated stubs defined in generated `.h` files.
 
 ### Java Objects and Garbage Collection
@@ -244,3 +246,9 @@ for more about the GN templates.
 
  * Python unit tests live in `jni_generator_tests.py`
  * A working demo app exists as `//base/android/jni_generator:sample_jni_apk`
+
+## Known Issues
+
+The `jni_generator` can cause build issues when source files that are used by
+JNI are moved due to how `include_dirs` are handled. This is tracked in
+[issue 964169](http://crbug.com/964169).
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