[media] videodev2.h.rst.exceptions: fix warnings

Changeset ab6343956f9c ("[media] V4L2: Add documentation for SDI timings
and related flags") added documentation for new V4L2 defines, but
it forgot to update videodev2.h.rst.exceptions to point to where
the documentation for those new values will be inside the book,
causing those warnings:

    Documentation/output/videodev2.h.rst:6: WARNING: undefined label: v4l2-dv-bt-std-sdi (if the link has no caption the label must precede a section header)
    Documentation/output/videodev2.h.rst:6: WARNING: undefined label: v4l2-dv-fl-first-field-extra-line (if the link has no caption the label must precede a section header)
    Documentation/output/videodev2.h.rst:6: WARNING: undefined label: v4l2-in-st-no-v-lock (if the link has no caption the label must precede a section header)
    Documentation/output/videodev2.h.rst:6: WARNING: undefined label: v4l2-in-st-no-std-lock (if the link has no caption the label must precede a section header)

Fixes: ab6343956f9c ("[media] V4L2: Add documentation for SDI timings and related flags")

Cc: Charles-Antoine Couret <charles-antoine.couret@nexvision.fr>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>