checkout.txt: document a common case that ignores ambiguation rules

checkout.txt: document a common case that ignores ambiguation rules

Normally we err on the safe side: if something can be seen as both an
SHA1 and a pathspec, we stop and scream. In checkout, there is one
exception added in 859fdab (git-checkout: improve error messages, detect
ambiguities. - 2008-07-23), to allow the common case "git checkout
branch". Let's document this exception.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>