R1:7482d260b08d

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…