This PR will apply the following changes on the [`__main__` module documentation](https://docs.python.org/3.7/library/__main__.html):

- correct the phrase "run as script" by "run from the file system" (as used in the [`runpy`](https://docs.python.org/3/library/runpy.html) documentation) since "run as script" does not mean the intended `python foo.py` but `python -m foo` (cf. [PEP 338](https://www.python.org/dev/peps/pep-0338/));
- replace the phrase "run with `-m`" by "run from the module namespace" (as used in the [`runpy`](https://docs.python.org/3/library/runpy.html) documentation) since the module can be equivalently run with `runpy.run_module('foo')` instead of `python -m foo`;
- make the block comment [PEP 8](https://www.python.org/dev/peps/pep-0008/#comments)-compliant (located before the `if` block, capital initialised, period ended);
- add a missing case for which a package's \_\_main\_\_.py is executed (when the package is run from the file system: `python foo/`).