trailer doc: <token> is a <key> or <keyAlias>, not both
The `--trailer` option takes a "<token>=<value>" argument, for example
--trailer "Acked-by=Bob"
And in this exampple it is understood that "Acked-by" is the <token>.
However, the user can use a shorter "ack" string by defining
configuration like
git config trailer.ack.key "Acked-by"
However, in the docs we define the above configuration as
trailer.<token>.key
so the <token> can mean either the longer "Acked-by" or the shorter
"ack".
Separate the two meanings of <token> into <key> and <keyAlias>, and
update the configuration syntax to say "trailer.<keyAlias>.key".
Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>