[Perl/perl5] 35a743: Document another case when the "not imported" warn...

[E. Choroba]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 35a7430ac2b6947e5c6e4f6a5abda7fe793b9012
      
https://github.com/Perl/perl5/commit/35a7430ac2b6947e5c6e4f6a5abda7fe793b9012
  Author: E. Choroba <chor...@matfyz.cz>
  Date:   2021-04-16 (Fri, 16 Apr 2021)

  Changed paths:
    M pod/perldiag.pod

  Log Message:
  -----------
  Document another case when the "not imported" warning is used

The minimal example:

  use strict;
  { our $x; }
  print $x;