[media] videobuf2-core.h: document enum vb2_memory

This enum was not documented. Document it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

include/media/videobuf2-core.h

@@ -20,6 +20,20 @@
 #define VB2_MAX_FRAME	(32)
 #define VB2_MAX_PLANES	(8)
 
+/**
+ * enum vb2_memory - type of memory model used to make the buffers visible
+ *	on userspace.
+ *
+ * @VB2_MEMORY_UNKNOWN:	Buffer status is unknown or it is not used yet on
+ *			userspace.
+ * @VB2_MEMORY_MMAP:	The buffers are allocated by the Kernel and it is
+ *			memory mapped via mmap() ioctl. This model is
+ *			also used when the user is using the buffers via
+ *			read() or write() system calls.
+ * @VB2_MEMORY_USERPTR:	The buffers was allocated in userspace and it is
+ *			memory mapped via mmap() ioctl.
+ * @VB2_MEMORY_DMABUF:	The buffers are passed to userspace via DMA buffer.
+ */
 enum vb2_memory {
 	VB2_MEMORY_UNKNOWN	= 0,
 	VB2_MEMORY_MMAP		= 1,