FileSystemStorage documentation spread out in different places

[nliberg]

The FileSystemStorage constructor parameters are not described in one place. Two of the parameters are described on an overview page:
​https://docs.djangoproject.com/en/dev/topics/files/#the-built-in-filesystem-storage-class
and the two other are described on the File storage API page:
​https://docs.djangoproject.com/en/dev/ref/files/storage/#the-filesystemstorage-class

It would probably be good if the API page described all four parameters.

[Tim Graham]

Would be best to put them on the reference doc only, I think. We should add a proper link to FileSystemStorage in the topic guide.

[Vishal Lal]

Issued ​Pull Request #2506. Verified documentation builds properly.

[Tim Graham <timograham@…>]

In ca93f02c621ea0f686e0bde3e4130e99116fd9d5:

Fixed #22347 -- Unified FileSystemStorage documentation.

Thanks nliberg for the report.

[Tim Graham <timograham@…>]

In 36d1e3725fd0b6a196ddb662fd1eb8bca638d38c:

[1.7.x] Fixed #22347 -- Unified FileSystemStorage documentation.

Thanks nliberg for the report.

Backport of ca93f02c62 from master