`if no packages are given` sounds a bit better than
`if none is given` when referring to `provided packages`.

Also mention that `pipenv install` with no provided packages
installs all packages from Pipfile. It is not obvious for
people new to pipenv whether pipenv is using Pipfile or
Pipfile.lock by default.

By default, `pipenv install` will install packages
based on the list in `Pipfile`, not `Pipfile.lock`.
`pipenv install --dev` is no exception.

I've tested this using pipenv 2018.7.1
with different package versions in `Pipfile`
and `Pipfile.lock`. After `pipenv install`,
the versions in `Pipfile` were the ones
actually installed.

The previous help text made it seem that
`pipenv install --dev` would install ONLY
dev-packages, which is not the case.

The current description is identical to the
one used in the documentation, which is
a lot clearer.