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