The documentation for the different versions (Plone 3, Plone 4, Plone 5) are organized in branches inside the Plone Documentation

The default branch points to the current version of Plone.

Documentation changes that are valid for multiple versions of Plone can be done by making multiple pull requests, or by cherry-picking which may be easier to do when branches are widely different.

When all this seems complicated, note in your pull request that you think this is valid for other versions of Plone as well, and the documentation team will take care.