bpo-44388: Update venv EnvBuilder.ensure_directories() docs. (GH-26663)

Ref: 

This changes the documentation for `EnvBuilder.ensure_directories(env_dir)` to match the actual behavior of that API call.

In particular, `ensure_directories()` is not affected by the state of the `upgrade` attribute, and will not cause an error to have existing directories whether or not the `clear` attribute is set.

This documentation change I believe should be valid to all python versions back to 3.6.

Automerge-Triggered-By: GH:vsajip