[Perl/perl5] b883c1: Document SvPV_shrink_to_cur

[Karl Williamson via perl5-changes]

  Branch: refs/heads/khw/docs_pr
  Home:   https://github.com/Perl/perl5
  Commit: b883c1499282ee1bef9809a1897a8cb9a033ac05
      
https://github.com/Perl/perl5/commit/b883c1499282ee1bef9809a1897a8cb9a033ac05
  Author: Karl Williamson <k...@cpan.org>
  Date:   2021-01-31 (Sun, 31 Jan 2021)

  Changed paths:
    M sv.h

  Log Message:
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  Document SvPV_shrink_to_cur