Clarify documentation for AImageReader_getHGBPFromHandle.

Explicitly state that the native handle taken by AImageReader_getHGBPFromHandle must have been
obtained from the llndk api : AImageReader_getWindowNativeHandle().

Bug: 110364143

Test: mm -j64

Change-Id: I75312b29b25cb79499a5c1a86f2d0ebbff2c3d67
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>