Updates module description, and uses the proper func for hex dump

As an user, it's important to know that using this module may result
a lost session because it must migrate to grep memory, but does not
migrate back.

The module also has its own hex dump routine, which is no longer
needed because we have a built-in Rex::Text.to_hex_dump
No need for the 2nd element
Use "unless"

Guidelines favor "unless".
Land #2034, @wchen-r7's patch for memory_grep