Documentation: SubmittingPatches: overhaul changelog description

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 <[email protected]>
Acked-by: David S. Miller <[email protected]>
Acked-by: Greg Kroah-Hartman <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Signed-off-by: Randy Dunlap <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>