Undocumented venv behavior

[brentonmallen1]

Issue description

I wouldn't really classify this as a bug but it seemed to be the best fit.

The behavior around installing dependencies into a .venv directory, if it exists, regardless of an unset PIPENV_VENV_IN_PROJECT environment variable, is not mentioned in the documentation. Seems to be only, officially, mentioned in a docstring:

pipenv/pipenv/environments.py

Line 306 in 0618fab

If unset (default), will use the .venv of project directory should it exist, otherwise

Expected result

A note about this behavior mentioned in the docs alongside the mention of the PIPENV_VENV_IN_PROJECT environment variable.

Actual result

N/A

Steps to replicate

N/A