Clarify format BLOB locking

Bug: 162571907
Test: n/a doc only change
Change-Id: I3ab26cee8a790067447781e13062c62fc3b7c01c
diff --git a/graphics/mapper/stable-c/include/android/hardware/graphics/mapper/IMapper.h b/graphics/mapper/stable-c/include/android/hardware/graphics/mapper/IMapper.h
index 0f6d146..0de88cd 100644
--- a/graphics/mapper/stable-c/include/android/hardware/graphics/mapper/IMapper.h
+++ b/graphics/mapper/stable-c/include/android/hardware/graphics/mapper/IMapper.h
@@ -241,7 +241,9 @@
      * acceptable.
      *
      * 1D buffers (width = size in bytes, height = 1, pixel_format = BLOB) must
-     * "lock in place". The buffers must be directly accessible via mapping.
+     * "lock in place" and behave similar to shared memory. That is, multiple threads or processes
+     * may lock the buffer for reading & writing and the results must follow the device's memory
+     * model.
      *
      * The client must not modify the content of the buffer outside of
      * @p accessRegion, and the device need not guarantee that content outside