docs: submitting-patches: clarify Acked-by and introduce "# Suffix"

Acked-by is typically used by maintainers. However, sometimes it is
useful to be able to accept the tag from other stakeholders that may not
have done a deep technical review or may not be kernel developers. For
instance:

  - People with domain knowledge, such as the original author of the
    code being modified.

  - Userspace-side reviewers for a kernel uAPI patch, like in DRM --
    see Documentation/gpu/drm-uapi.rst:

    > The userspace-side reviewer should also provide an Acked-by on the
    > kernel uAPI patch indicating that they believe the proposed uAPI
    > is sound and sufficiently documented and validated for userspace's
    > consumption.

  - Key users of a feature, such as in [1].

Thus clarify that Acked-by may be used by other stakeholders (but most
commonly by maintainers).

Since, in these cases, it may be confusing why an Acked-by is/was
provided, allow and suggest to provide a "# Suffix" explaining it.

The "# Suffix" for Acked-by is already being used to clarify what part
of the patch a maintainer is acknowledging, thus also mention "# Suffix"
in the relevant paragraph.

Link: https://lore.kernel.org/rust-for-linux/CANiq72m4fea15Z0fFZauz8N2madkBJ0G7Dc094OwoajnXmROOA@mail.gmail.com/

 [1]
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250112152946.761150-2-ojeda@kernel.org