SubmittingPatches: clarify the expected commit log description

SubmittingPatches: clarify the expected commit log description

Earlier, 47afed5 (SubmittingPatches: itemize and reflect upon well written
changes, 2009-04-28) added a discussion on the contents of the commit log
message, but the last part of the new paragraph didn't make much sense.
Reword it slightly to make it more readable.

Update the "quicklist" to clarify what we mean by "motivation" and
"contrast".  Also mildly discourage external references.

Signed-off-by: Junio C Hamano <redacted>