Modify javadoc for onWindowShown

onWindowShown is more like it should called after mWindow.show()
in InputMethodService. Considering the compatibility problem, just
make its javadoc clearer to the IME developers rather than
change the calling order.

Bug: 72922821
Test: N/A

Change-Id: Ibfe20f40a65475f39c8e79d10e2c494e212cf054
Signed-off-by: tiansiming [田思明] <tiansiming@xiaomi.com>