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>

Documentation/git-interpret-trailers.txt

@@ -117,7 +117,7 @@ OPTIONS
 	and applies to all '--trailer' options until the next occurrence of
 	'--where' or '--no-where'. Upon encountering '--no-where', clear the
 	effect of any previous use of '--where', such that the relevant configuration
-	variables are no longer overridden. Possible values are `after`,
+	variables are no longer overridden. Possible placements are `after`,
 	`before`, `end` or `start`.
 
 --if-exists <action>::

builtin/interpret-trailers.c

@@ -97,7 +97,7 @@ int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "in-place", &opts.in_place, N_("edit files in place")),
 		OPT_BOOL(0, "trim-empty", &opts.trim_empty, N_("trim empty trailers")),
 
-		OPT_CALLBACK(0, "where", NULL, N_("action"),
+		OPT_CALLBACK(0, "where", NULL, N_("placement"),
 			     N_("where to place the new trailer"), option_parse_where),
 		OPT_CALLBACK(0, "if-exists", NULL, N_("action"),
 			     N_("action if trailer already exists"), option_parse_if_exists),