Echo commands without arguments doesn't print existing variables - update help message

   If the command is executed w/o arguments then the command will print
   all the existing variables with their values in the name=value format.

[standalone@localhost:9990 /] set foo=bar
[standalone@localhost:9990 /] echo $foo
bar
[standalone@localhost:9990 /] echo

[standalone@localhost:9990 /]

[standalone@localhost:9990 /] set foo=bar
[standalone@localhost:9990 /] echo $foo
bar
[standalone@localhost:9990 /] echo
foo=bar
[standalone@localhost:9990 /]