trailer doc: narrow down scope of --where and related flags

trailer doc: narrow down scope of --where and related flags

The wording "all configuration variables" is misleading (the same could
be said to the descriptions of the "--[no-]if-exists" and the
"--[no-]if-missing" options).  Specifying --where=value overrides only
the trailer.where variable and applicable trailer.<token>.where
variables, and --no-where stops the overriding of these variables.
Ditto for the other two with their relevant configuration variables.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>