Merge branch 'se/doc-checkout-ours-theirs' into maint

A "rebase" replays changes of the local branch on top of something
else, as such they are placed in stage #3 and referred to as
"theirs", while the changes in the new base, typically a foreign
work, are placed in stage #2 and referred to as "ours".  Clarify
the "checkout --ours/--theirs".

* se/doc-checkout-ours-theirs:
checkout: document subtlety around --ours/--theirs

diff --cc Documentation/git-checkout.txt
Simple merge