BUG #15396: initdb emits wrong comment for range foreffective_io_concurrency

The following bug has been logged on the website:

Bug reference:      15396
Logged by:          James Robinson
Email address:      james@jlr-photo.com
PostgreSQL version: 11beta3
Operating system:   OSX
Description:

inidb in 11 beta4 emits a postgresql.conf with the following:

  ...
  # - Asynchronous Behavior -

  #effective_io_concurrency = 0        # 1-1000; 0 disables prefetching
  ...

But un-commenting and changing to any value other than zero produces:

  2018-09-23 15:57:15.960 GMT [33648] LOG:  1 is outside the valid range for
parameter "effective_io_concurrency" (0 .. 0)

Something up here? If not a tunable knob, why describe it and / or have the
misleading comment?

beta4 Installed via macports.