summary: update nspawn description string a bit

nspawn as it is now is a generally useful tool, hence let's drop the
comments about it being useful for debug and so on only.

The new wording just makes the first sentence of the main page also the
summary.

src/nspawn/nspawn.c

@@ -208,7 +208,7 @@ static void help(void) {
         (void) pager_open(false, false);
 
         printf("%s [OPTIONS...] [PATH] [ARGUMENTS...]\n\n"
-               "Spawn a minimal namespace container for debugging, testing and building.\n\n"
+               "Spawn a command or OS in a light-weight container.\n\n"
                "  -h --help                 Show this help\n"
                "     --version              Print version string\n"
                "  -q --quiet                Do not show status information\n"