dma-buf: Add "dma-buf" to title of documentation

To make it easier to find the dma-buf documentation when looking through
tables-of-contents etc., put the name "dma-buf" in the title.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20230129115917.1229529-1-j.neuschaefer@gmx.netSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/dma-buf.rst

-Buffer Sharing and Synchronization
-==================================
+Buffer Sharing and Synchronization (dma-buf)
+============================================
 
 The dma-buf subsystem provides the framework for sharing buffers for
 hardware (DMA) access across multiple device drivers and subsystems, and