BUG#19322 Added help defintion for --skip-external locking to mysqld --help --verbose

sql/mysqld.cc:
  Added --skip-external-locking to the --external-locking help.

sql/mysqld.cc

@@ -4812,7 +4812,8 @@ Disable with --skip-bdb (will save memory).",
    0, GET_BOOL, NO_ARG, 0, 0, 0, 0, 0, 0},
   {"exit-info", 'T', "Used for debugging;  Use at your own risk!", 0, 0, 0,
    GET_LONG, OPT_ARG, 0, 0, 0, 0, 0, 0},
-  {"external-locking", OPT_USE_LOCKING, "Use system (external) locking.  With this option enabled you can run myisamchk to test (not repair) tables while the MySQL server is running.",
+  {"external-locking", OPT_USE_LOCKING, "Use system (external) locking (disabled by default). With this option enabled you can run myisamchk to test (not repair) tables while the MySQL server is running. \
+Disable with --skip-external-locking.",
    (gptr*) &opt_external_locking, (gptr*) &opt_external_locking,
    0, GET_BOOL, NO_ARG, 0, 0, 0, 0, 0, 0},
   {"flush", OPT_FLUSH, "Flush tables to disk between SQL commands.", 0, 0, 0,