"pipenv lock --dev --requirements" doesn't include default packages

[asmaier]

Issue description

pipenv lock --dev --requirements > requirements.txt 

generates a requirement.txt that doesn't include the default packages. It only includes the development packages.

Expected result

According to the documentation the flag --dev means

Install both develop and default packages.

(see https://docs.pipenv.org/en/latest/#pipenv-lock)

So either the documentation should be changed or the behaviour of pipenv lock --dev should be fixed.

[frostming]

The option's help string is not accurate. Want a PR to fix it.

Related discussion: #3503 #3316 , close this one to reduce redundancy.