Agreed, the docs don't need to mention PATHEXT.

I don't have a machine to hand with Python 3.6 not installed, to check, but I believe the launcher is installed by default (although as you say it can be deselected - and it's definitely described as "recommended") whereas adding Python to PATH is deselected by default. So "py" is more likely to work on a typical installation than "python".

As I say, though, I have no feel here for the right level of detail. Maybe something like

"""
To execute the venv command, use

    python -m venv

Unix installations typically name the Python 3.x executable "python3", and on Windows the "py" launcher is generally available even if the Python interpreter is not on your PATH, so you may need to substitute the appropriate means of invoking Python in place of the "python" command above.
"""

But that's (to my mind) still wordy and complex - and the new users I've spoken to already find the docs too full of confusing detail.

Ideally, I'd prefer it if "Invoking Python" were described once and for all in a separate section of the docs (i.e., the "Python Setup and Usage" manual) and everywhere else in the official documentation we simply used "python", with no qualifications, on the understanding that this stands for "whatever you use on your PC to run Python". I suspect that would generate complaints from a lot of people, though, as most people hit the doc pages via direct links, not by reading the manuals in order...