doc: Add description of "filename" for pg_walsummary

doc: Add description of "filename" for pg_walsummary

This command requires an input file (WAL summary file), that has to be
specified without an option name.  The shape of the command and how to
use this parameter is implied in its synopsis.  However, this page
lacked a description of the parameter.  Listing parameters that do
not require an option is a common practice across the docs.  See for
example pg_dump, pg_restore, etc.

Author: Peter Smith <smithpb2250@gmail.com>
Discussion: https://postgr.es/m/CAHut+PtbQi8Dw_0upS9dd=Oh9OqfOdAo=0_DOKG=YSRT_a+0Fw@mail.gmail.com