Changed the dest_prefix possible values and added documentation

* I realized the previous dest_prefix=no value was breaking the idea
of having dest as a directory only (as stated in the documentation).
It can now be used with dest_prefix=simple, which only use the dest
option directory and the filename without its path.
Which means we can't chose the destination filename anymore, but it's
not disturbing in my use cases.
Using the dest_prefix=simple option with more than one host means that
you have to define the dest and/or src options with non static values
if you don't want to overwrite the same file.

* Updated the documentation