Clarify docs for public_baseurl

[dbkr]

This is leading to problems with people upgrading to clients that
support MSC1730 because people have this misconfigured, so try
to make the docs completely unambiguous.

NB. In a world without ACME support I probably would have also remove the 8448 from the example URL as this would never have worked, being a self-signed cert, but I guess now this is a plausible default, although it still feels like it's directing you towards it being the internal URL. wdyt?

[codecov-io]

Codecov Report

Merging #4458 into develop will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop   #4458      +/-   ##
==========================================
+ Coverage    74.79%   74.8%   +0.01%     
==========================================
  Files          336     336              
  Lines        33997   33997              
  Branches      5527    5527              
==========================================
+ Hits         25429   25433       +4     
+ Misses        7002    7001       -1     
+ Partials      1566    1563       -3