Clearer error message when Change-Id line not found

Improve the error message that is reported when a Change-Id
line is found in the commit message, but not in the footer.

Now the error message makes it clearer that the Change-Id is
expected to be found in last paragraph.

Also updated the documentation with the same clarification,
and added a cross-reference link to the Signed-Off-By page.

Change-Id: I0a8329edb89aea32f02ba285d292652bf6c0ee55
2 files changed