Provide more clear flask -e, --env-file flag desription

[differencialx]

Currently the description for flask cli command -e, --env-file flag is misleading. I understand that this functionality is covered and perfectly explained in docs, but in case you are not familiar with documentation you will just use flask --help command to read which flags are allowed.

This PR just adds few sentences to make -e, --env-file flag description more clear and exhaustive. Before: After

Initial PR with for -e, --env-file flag: https://github.com/pallets/flask/pull/4646