Doc updates.
* je/doc-checkout:
doc: git-checkout: clarify restoring files section
doc: git-checkout: split up restoring files section
doc: git-checkout: deduplicate --detach explanation
doc: git-checkout: clarify `-b` and `-B`
doc: git-checkout: clarify `git checkout <branch>`
doc: git-checkout: clarify ARGUMENT DISAMBIGUATION
doc: git-checkout: clarify intro sentence