Documentation: simplify and clarify DCO contribution example language

Long long ago, in a more innocent time, Greg wrote the clarification for
how the DCO should work and that you couldn't make anonymous
contributions, because the sign-off needed to be something we could
check back with.

It was 2006, and nobody reacted to the wording, the whole Facebook 'real
name' controversy was a decade in the future, and nobody even thought
about it. And despite the language, we've always accepted nicknames and
that language was never meant to be any kind of exclusionary wording.

In fact, even when it became a discussion in other adjacent projects,
apparently nobody even thought to just clarify the language in the
kernel docs, and instead we had projects like the CNCF that had long
discussions about it, and wrote their own clarifications [1] of it.

Just simplify the wording to the point where it shouldn't be causing
unnecessary angst and pain, or scare away people who go by preferred
naming.

Link: https://github.com/cncf/foundation/blob/659fd32c86dc/dco-guidelines.md [1]
Fixes: af45f32d25cc ("We can not allow anonymous contributions to the kernel")
Acked-by: Greg KH <gregkh@linuxfoundation.org>
Acked-by: Michael Dolan <mdolan@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>