Update docs to clarify how to pass job options in client mode

Update docs to clarify how to pass job options in client mode

When run in client mode, fio does not pass any job options specified on
the command line to the fio server. When run in client mode, all job
options must be specified via local or remote job files. Update the docs
to indicate this to avoid end-user confusion.

Fixes #1629

Signed-off-by: Nick Neumann nick@pcpartpicker.com