rev-parse: clarify documentation of $name@{upstream} syntax

rev-parse: clarify documentation of $name@{upstream} syntax

"git rev-parse" interprets string in string@{upstream} as a name of
a branch not a ref. For example, refs/heads/master@{upstream} looks
for an upstream branch that is merged by git-pull to ref
refs/heads/refs/heads/master not to refs/heads/master.

However the documentation could mislead a user to believe that the
string is interpreted as ref.

Signed-off-by: Kacper Kornet <draenog@pld-linux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>