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.

man/systemd-nspawn.xml

@@ -24,7 +24,7 @@
 
   <refnamediv>
     <refname>systemd-nspawn</refname>
-    <refpurpose>Spawn a namespace container for debugging, testing and building</refpurpose>
+    <refpurpose>Spawn a command or OS in a light-weight container</refpurpose>
   </refnamediv>
 
   <refsynopsisdiv>
@@ -55,7 +55,7 @@
     <para><command>systemd-nspawn</command> may be invoked on any directory tree containing an operating system tree,
     using the <option>--directory=</option> command line option. By using the <option>--machine=</option> option an OS
     tree is automatically searched for in a couple of locations, most importantly in
-    <filename>/var/lib/machines</filename>, the suggested directory to place container images installed on the
+    <filename>/var/lib/machines</filename>, the suggested directory to place OS container images installed on the
     system.</para>
 
     <para>In contrast to <citerefentry

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"