yanglint DOC added information about submodules

... to the print command.
diff --git a/tools/lint/cmd_print.c b/tools/lint/cmd_print.c
index 4157cc8..0017044 100644
--- a/tools/lint/cmd_print.c
+++ b/tools/lint/cmd_print.c
@@ -34,7 +34,8 @@
     printf("Usage: print [-f (yang | yin | tree [-q -P PATH -L LINE_LENGTH ] | info [-q -P PATH])]\n"
             "            [-o OUTFILE] [<module-name1>[@revision]] ...\n"
             "                  Print a schema module. The <module-name> is not required\n"
-            "                  only in case the -P option is specified.\n\n"
+            "                  only in case the -P option is specified. For yang, yin and tree\n"
+            "                  formats, a submodule can also be printed.\n\n"
             "  -f FORMAT, --format=FORMAT\n"
             "                  Print the module in the specified FORMAT. If format not\n"
             "                  specified, the 'tree' format is used.\n"