Documentation: clarify tutorial pull/merge discussion

Documentation: clarify tutorial pull/merge discussion

Attempt to clarify somewhat the difference between pull and merge,
and give a little more details on the pull syntax.

I'm still not happy with this section: the explanation of the origin
branch isn't great, but maybe that should be left alone pending the
use-separate-remotes change; and we need to explain how to set up a
public repository and push to it.

Signed-off-by: J. Bruce Fields <redacted>
Signed-off-by: Junio C Hamano <redacted>