backout: document return code of merge conflict

--HG--
branch : stable
backout: correct commit status of no changes made (BC) (issue4190)

If backout generated no changes to commit, it showed wrong status, "changeset
<target> backs out changeset <target>", and raised TypeError with -v option.

This changes the return code to 1, which is the same as "hg commit" and
"hg rebase".

--HG--
branch : stable
Added tag 2.9.2 for changeset 3f83fc5cfe71

--HG--
branch : stable
Added signature for changeset 3f83fc5cfe71

--HG--
branch : stable