dma-buf: Documentation update for Kconfig select

As per Linus' comment, dma-buf Kconfig entry shouldn't have an option
text, but should be selected by the subsystems that use it.

Add this information in the documentation as well.
Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

Documentation/dma-buf-sharing.txt

@@ -219,6 +219,10 @@ NOTES:
    If the exporter chooses not to allow an attach() operation once a
    map_dma_buf() API has been called, it simply returns an error.
 
+Miscellaneous notes:
+- Any exporters or users of the dma-buf buffer sharing framework must have
+  a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
+
 References:
 [1] struct dma_buf_ops in include/linux/dma-buf.h
 [2] All interfaces mentioned above defined in include/linux/dma-buf.h