trailer: trailer location is a place, not an action

trailer: trailer location is a place, not an action

Fix the help text to say "placement" instead of "action" because the
values are placements, not actions.

While we're at it, tweak the documentation to say "placements" instead
of "values", similar to how the existing language for "--if-exists" uses
the word "action" to describe both the syntax (with the phrase
"--if-exists <action>") and the possible values (with the phrase
"possible actions").

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