Improve documentation for unintended usage of python_version

[iuriguilherme]

I've read #1050 but since it's closed I'm opening another issue for documentation improvement.

The warning messages are misleading about why you can't use "greater than" characters in the python_version:

Warning: Python >= 3.6 was not found on your system...

Warning: Your Pipfile requires python_version >= 3.6, but you are using 3.9.2 (/home/user/.local/share/v/i/bin/python).

Perhaps a message in the lines of multiple versions of python are not allowed - please don't use '> 3.6' but '3.6' or something like that would make more sense.

I'm sorry I can't contribute with a proper documentation right now, I just hope I was able to state what the issue is.