Document that you should increase the max client file upload size in reverse proxy docs

[anoadragon453]

We should specify the client_max_body_size nginx option and equivalent for other reverse proxies in https://github.com/matrix-org/synapse/blob/master/docs/reverse_proxy.md so people don't get caught up when they can't upload large images (and Synapse doesn't report an error).

Also client developers should interpret the 413 from the reverse proxy as the image is too large...

[Graphicscore]

I second this, this was exactly what I needed +1

[dklimpel]

This issue can be closed?
See #10122 (comment)

There is a note in reverse proxy doc and config settings.

[clokep]

Seems like all the relevant info is there. Thanks!