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>

builtin/interpret-trailers.c

@@ -14,7 +14,7 @@
 
 static const char * const git_interpret_trailers_usage[] = {
 	N_("git interpret-trailers [--in-place] [--trim-empty]\n"
-	   "                       [(--trailer <token>[(=|:)<value>])...]\n"
+	   "                       [(--trailer (<key>|<keyAlias>)[(=|:)<value>])...]\n"
 	   "                       [--parse] [<file>...]"),
 	NULL
 };