Fix documentation of initdb --show option

It wasn't in the documentation at all (even though we document all the
other debugging-like options).  Also, change the --help output to show
that it exits after showing, similar to other options.

doc/src/sgml/ref/initdb.sgml

@@ -525,6 +525,18 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry id="app-initdb-option-show">
+      <term><option>-s</option></term>
+      <term><option>--show</option></term>
+      <listitem>
+       <para>
+        Show internal settings and exit, without doing anything else.  This
+        can be used to debug the <application>initdb</application>
+        installation.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="app-initdb-option-sync-method">
       <term><option>--sync-method=<replaceable>method</replaceable></option></term>
       <listitem>

src/bin/initdb/initdb.c

@@ -2481,7 +2481,7 @@ usage(const char *progname)
 	printf(_("  -n, --no-clean            do not clean up after errors\n"));
 	printf(_("  -N, --no-sync             do not wait for changes to be written safely to disk\n"));
 	printf(_("      --no-instructions     do not print instructions for next steps\n"));
-	printf(_("  -s, --show                show internal settings\n"));
+	printf(_("  -s, --show                show internal settings, then exit\n"));
 	printf(_("      --sync-method=METHOD  set method for syncing files to disk\n"));
 	printf(_("  -S, --sync-only           only sync database files to disk, then exit\n"));
 	printf(_("\nOther options:\n"));