5669: pytester: add docstrings for Testdir.copy_example

[martbln]

• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Add yourself to AUTHORS in alphabetical order;

[codecov]

Codecov Report

Merging #5719 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5719   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files         117      117           
  Lines       25864    25864           
  Branches     2498     2498           
=======================================
  Hits        24873    24873           
  Misses        690      690           
  Partials      301      301

Impacted Files	Coverage Δ
src/_pytest/pytester.py	90.96% <ø> (ø)	⬆️

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[nicoddemus]

Thanks!

[martbln]

Thanks!
👍🏻