media: mediactl docs: Fix licensing message

Right now, it mentions two SPDX headers that don't exist inside the Kernel:
	GFDL-1.1-or-later
And an exception:
	no-invariant-sections

While it would be trivial to add the first one, there's no way,
currently, to distinguish, with SPDX, between a free and a non-free
document under GFDL.

Free documents with GFDL should not have invariant sections.

There's an open issue at SPDX tree waiting for it to be solved.
While we don't have this issue closed, let's just replace by a
free-text license, and add a TODO note to remind us to revisit it
later.

Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>