Rewrite file documentation to be chunk-aware

Summary:
Ref T7149. We can simplify configuration somewhat by removing the upload limit setting, now that we support arbitrarily large files.

• Merge configuration documentation.
• Tell users to set things to at least 32MB. This is 8MB maximum one-shot file + 4x headroom. Chunk sizes are 4MB.

Test Plan:

• Faked all the setup warnings.
• Read documentation.
• Uploaded some files.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12083