[Perl/perl5] 5bef5f: Document SvPV_renew

[Karl Williamson via perl5-changes]

  Branch: refs/heads/smoke-me/khw-pr_api3
  Home:   https://github.com/Perl/perl5
  Commit: 5bef5f050663d79ae4d559743f52340e06ee694c
      
https://github.com/Perl/perl5/commit/5bef5f050663d79ae4d559743f52340e06ee694c
  Author: Karl Williamson <k...@cpan.org>
  Date:   2020-09-28 (Mon, 28 Sep 2020)

  Changed paths:
    M sv.h

  Log Message:
  -----------
  Document SvPV_renew