Update the description in file.info.yml

Problem/Motivation

Currently, file.info.yml says this:
description: 'Defines a file field type.'

In D8, it does an awful lot more than that. ;) It's also providing:

• The File entity.
• The managed_file form element.
• File usage tracking.
• The REST file upload resource.
• ...

I doubt most site builders care about most of this, but I'm sure some of them would care about some of it. ;)

Proposed resolution

Update the file.info.yml description.

Remaining tasks

1. Figure out what subset of what file.module does that most site builders looking at the .info.yml file or /admin/modules would care about.
2. #include bikeshed.h
3. Write the patch
4. RTBC
5. Commit

User interface changes

More accurate description of the file module on the /admin/modules page.

API changes

None.

Data model changes

None.