dma-buf: Document that dma-buf size is fixed

The fact that the size of dma-buf is invariant over the lifetime of the
buffer is mentioned in the comment of 'dma_buf_ops.mmap', but is not
documented at where the info is defined. Add the missing documentation.
Signed-off-by: Jianxin Xiong <jianxin.xiong@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1605044477-51833-7-git-send-email-jianxin.xiong@intel.com

include/linux/dma-buf.h

@@ -273,7 +273,7 @@ struct dma_buf_ops {
 
 /**
  * struct dma_buf - shared buffer object
- * @size: size of the buffer
+ * @size: size of the buffer; invariant over the lifetime of the buffer.
  * @file: file pointer used for sharing buffers across, and for refcounting.
  * @attachments: list of dma_buf_attachment that denotes all devices attached,
  *               protected by dma_resv lock.
@@ -405,7 +405,7 @@ struct dma_buf_attachment {
  * @exp_name:	name of the exporter - useful for debugging.
  * @owner:	pointer to exporter module - used for refcounting kernel module
  * @ops:	Attach allocator-defined dma buf ops to the new buffer
- * @size:	Size of the buffer
+ * @size:	Size of the buffer - invariant over the lifetime of the buffer
  * @flags:	mode flags for the file
  * @resv:	reservation-object, NULL to allocate default one
  * @priv:	Attach private data of allocator to this buffer