[Perl/perl5] 4c29eb: add a brief introduction to the IO SV type

[Tony Cook via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 4c29eb71ff3f0996f907fcc484a329bb673f3606
      
https://github.com/Perl/perl5/commit/4c29eb71ff3f0996f907fcc484a329bb673f3606
  Author: Tony Cook <t...@develop-help.com>
  Date:   2020-11-23 (Mon, 23 Nov 2020)

  Changed paths:
    M pod/perlguts.pod

  Log Message:
  -----------
  add a brief introduction to the IO SV type