scripts: west build: slight --help tweak

We have an open request to make the help for the -t option a little
easier. Try to do that without adding too much length to the short
help.

Fixes: #16202
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

scripts/west_commands/build.py

@@ -94,7 +94,8 @@ class Build(Forceable):
                             " Otherwise the default build directory is " +
                             "created and used.")
         parser.add_argument('-t', '--target',
-                            help='''Build system target to run''')
+                            help='''Build system target ("usage"
+                            for more info; and "help" for a list)''')
         parser.add_argument('-p', '--pristine', choices=['auto', 'always',
                             'never'], action=AlwaysIfMissing, nargs='?',
                             help='''Control whether the build folder is made