docs: clarify when venv needs to be re-setup
docs: clarify when venv needs to be re-setup

When dependencies change or a new plugin is introduced, the venv needs
to be re-created.

Partially addresses issue #4368.

letsencrypt/letsencrypt,jsha/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt,stweil/letsencrypt,lmcro/letsencrypt,stweil/letsencrypt,jsha/letsencrypt