media: v4l docs: move some cross-reference identifiers

Sphinx doesn't allow placing those cross-reference identifies
everywhere. Misplacing them cause those warnings:

Documentation/userspace-api/media/v4l/hist-v4l2.rst:818: WARNING: undefined label: control-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/hist-v4l2.rst:853: WARNING: undefined label: reserved-formats (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst:47: WARNING: undefined label: reserved-formats (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/subdev-formats.rst:39: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/subdev-formats.rst:53: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/subdev-formats.rst:67: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/subdev-formats.rst:83: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/subdev-formats.rst:97: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/subdev-formats.rst:140: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-decoder-cmd.rst:75: WARNING: undefined label: decoder-cmds (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-dqevent.rst:234: WARNING: undefined label: control-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-g-dv-timings.rst:156: WARNING: undefined label: dv-bt-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst:41: WARNING: undefined label: ctrl-class (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst:246: WARNING: undefined label: ctrl-class (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst:269: WARNING: undefined label: ctrl-class (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-g-modulator.rst:100: WARNING: undefined label: modulator-txsubchans (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-g-tuner.rst:119: WARNING: undefined label: tuner-rxsubchans (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst:167: WARNING: undefined label: control-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst:245: WARNING: undefined label: control-flags (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-queryctrl.rst:396: WARNING: undefined label: ctrl-class (if the link has no caption the label must precede a section header)
Documentation/userspace-api/media/v4l/vidioc-subdev-enum-mbus-code.rst:73: WARNING: undefined label: v4l2-subdev-mbus-code-flags (if the link has no caption the label must precede a section header)

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

media: docs: uAPI: fix table output in LaTeX/PDF format

There are lots of tables that are not properly displayed
in LaTeX/PDF.

Fix the tablecolumns, add longtable where needed and LaTeX
formatting macros, in order to address such issues.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

media: docs: make V4L documents more compatible with Sphinx 3.1+

Sphinx 3.x broke support for the cdomain.py extension, as the
c domain code was rewritten. Due to that, the c tags need to
be re-written, in order to use the new c domain notation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

media: docs: use the new SPDX header for GFDL-1.1 on *.rst files

SPDX v3.10 gained support for GFDL-1.1 with no invariant sections:

https://spdx.org/licenses/GFDL-1.1-no-invariants-or-later.html

So, remove the license text, replacing them by this new SPDX
license.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee3862 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>