[Perl/perl5] 6e7a78: av.h - document newAV_alloc_xz

[Richard Leach via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 6e7a7813c390d2121876b9f8af4f37102e8d51fa
      
https://github.com/Perl/perl5/commit/6e7a7813c390d2121876b9f8af4f37102e8d51fa
  Author: Richard Leach <richardle...@users.noreply.github.com>
  Date:   2021-07-29 (Thu, 29 Jul 2021)

  Changed paths:
    M av.h

  Log Message:
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  av.h - document newAV_alloc_xz