[Perl/perl5] b1decf: Document gv_fetchfile(_flags)?

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: b1decff385f198a18c063aa3815c47f77440aab6
      
https://github.com/Perl/perl5/commit/b1decff385f198a18c063aa3815c47f77440aab6
  Author: Karl Williamson <k...@cpan.org>
  Date:   2020-12-27 (Sun, 27 Dec 2020)

  Changed paths:
    M embed.fnc
    M gv.c

  Log Message:
  -----------
  Document gv_fetchfile(_flags)?