Add Completion docs re filename expansion default

Add Completion docs re filename expansion default

Since the fact that the Completion plugin *disables* an already
on feature, added a note to that effect.  The current behavior
is: no Completion plugin loaded then you'll get the Term::ReadLine
default of filename expansion (unless disabled at the Term::ReadLine
level), and with Completion plugin loaded any fallback Term::ReadLine
expansion will not happen.  To use Term::ReadLine completion again,
just set the do_readline_filename_completion attribute to 1.