The docstring for load_dotenv was missing a word, rendering it confusing.
This commits modifies it for clarity.

This modifies Dotenv.set_as_environment_variables to return False
if we have not discovered any environment variables via either
`dotenv_path` or `stream`.

The return value gets passed through to `load_dotenv`, so this can be
used to determine if `dotenv.load_dotenv` was able to set anything.

Closes theskumar#321