Add --bip6 to the dockerd manpage

[robmry]

- What I did

• related to Add --bip6 to dockerd cmdline ref #5597

Copied manpage changes from moby/moby@0b5b1db .... accidentally missed from the PR above.

- How I did it

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.50%. Comparing base (2369935) to head (ae54c9d).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5655   +/-   ##
=======================================
  Coverage   59.50%   59.50%           
=======================================
  Files         346      346           
  Lines       29369    29369           
=======================================
  Hits        17475    17475           
  Misses      10920    10920           
  Partials      974      974           

[thaJeztah]

LGTM, thanks!! I can close that "reminder" tab now ❤️