Documentation: SubmittingPatches: overhaul changelog description

Maintainers often repeat the same feedback on poorly written
changelogs - describe the problem, justify your changes, quantify
optimizations, describe user-visible changes - but our documentation
on writing changelogs doesn't include these things.  Fix that.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>