doc-rst: mmap: Add ioctl cross references

There are lots of ioctls mentioned there that aren't cross-referenced.

Convert the const to cross references. That makes it visually
better, and improves navigation along the document.

While here, remove bad whitespaces.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/linux_tv/media/v4l/mmap.rst

@@ -224,37 +224,38 @@ mapped buffers, then to start capturing and enter the read loop. Here
 the application waits until a filled buffer can be dequeued, and
 re-enqueues the buffer when the data is no longer needed. Output
 applications fill and enqueue buffers, when enough buffers are stacked
-up the output is started with ``VIDIOC_STREAMON``. In the write loop,
-when the application runs out of free buffers, it must wait until an
-empty buffer can be dequeued and reused.
-
-To enqueue and dequeue a buffer applications use the
-:ref:`VIDIOC_QBUF` and
-:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The status of a buffer being
-mapped, enqueued, full or empty can be determined at any time using the
-:ref:`VIDIOC_QUERYBUF` ioctl. Two methods exist to
-suspend execution of the application until one or more buffers can be
-dequeued. By default ``VIDIOC_DQBUF`` blocks when no buffer is in the
-outgoing queue. When the ``O_NONBLOCK`` flag was given to the
-:ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns
-immediately with an ``EAGAIN`` error code when no buffer is available. The
-:ref:`select() <func-select>` or :ref:`poll() <func-poll>` functions
-are always available.
+up the output is started with :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`.
+In the write loop, when the application runs out of free buffers, it
+must wait until an empty buffer can be dequeued and reused.
+
+To enqueue and dequeue a buffer applications use the :ref:`VIDIOC_QBUF`
+and :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The status of a buffer
+being mapped, enqueued, full or empty can be determined at any time
+using the :ref:`VIDIOC_QUERYBUF` ioctl. Two methods exist to suspend
+execution of the application until one or more buffers can be dequeued.
+By default :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` blocks when no buffer is
+in the outgoing queue. When the ``O_NONBLOCK`` flag was given to the
+:ref:`open() <func-open>` function, :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
+returns immediately with an ``EAGAIN`` error code when no buffer is
+available. The :ref:`select() <func-select>` or :ref:`poll()
+<func-poll>` functions are always available.
 
 To start and stop capturing or output applications call the
-:ref:`VIDIOC_STREAMON` and
-:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctl. Note
-``VIDIOC_STREAMOFF`` removes all buffers from both queues as a side
-effect. Since there is no notion of doing anything "now" on a
-multitasking system, if an application needs to synchronize with another
-event it should examine the struct :ref:`v4l2_buffer <v4l2-buffer>`
-``timestamp`` of captured or outputted buffers.
+:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` and :ref:`VIDIOC_STREAMOFF
+<VIDIOC_STREAMON>` ioctl. Note :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
+removes all buffers from both queues as a side effect. Since there is
+no notion of doing anything "now" on a multitasking system, if an
+application needs to synchronize with another event it should examine
+the struct ::ref:`v4l2_buffer <v4l2-buffer>` ``timestamp`` of captured
+or outputted buffers.
 
 Drivers implementing memory mapping I/O must support the
-``VIDIOC_REQBUFS``, ``VIDIOC_QUERYBUF``, ``VIDIOC_QBUF``,
-``VIDIOC_DQBUF``, ``VIDIOC_STREAMON`` and ``VIDIOC_STREAMOFF`` ioctl,
-the :ref:`mmap() <func-mmap>`, :ref:`munmap() <func-munmap>`, :ref:`select() <func-select>` and
-:ref:`poll() <func-poll>` function. [3]_
+:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, :ref:`VIDIOC_QUERYBUF
+<VIDIOC_QUERYBUF>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_DQBUF
+<VIDIOC_QBUF>`, :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
+and :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls, the :ref:`mmap()
+<func-mmap>`, :ref:`munmap() <func-munmap>`, :ref:`select()
+<func-select>` and :ref:`poll() <func-poll>` function. [3]_
 
 [capture example]