Unify docstrings of Gnus summary's copy/move/crosspost article functions

* lisp/gnus/gnus-sum.el (gnus-summary-copy-article)
(gnus-summary-crosspost-article): Make docstrings refer to
gnus-summary-move-article.
1 job for master in 62 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #4322
test-all

01:02:21

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
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make[2]: Leaving directory '/builds/emacs/emacs/test'
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make[1]: Leaving directory '/builds/emacs/emacs/test'
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make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1