nginx fastcgi updates

[mmattel]

This is an update of the nginx documentation to owncloud/core#29328 (Big file download from SMB via fast networks eats up memory) implementing the ability to:

• set fastcgi_buffers 8 4k; correctly
• adding fastcgi_ignore_headers X-Accel-Buffering;
• removing default fastcgi_buffering off;
• adding notes to document the reason

Imho this is complete now.
When merged, we can close owncloud/core#29328
Will also do a backport.

@PVince81 @jvillafanez

[jvillafanez]

LGTM

[settermjd]

@mmattel @jvillafanez thanks for making this change. It's been backported to stable10 as well.

[mmattel]

For referencing: owncloud/core#29400 (Disable nginx buffering for file downloads)

[settermjd]

@mmattel I don't quite understand your comment. Do you want that PR referenced in the changed documentation?

[mmattel]

Following intention (no further action necessary):
There is a core issue (Testing 10.0.4RC1 Changelog) which refrences core 29400 as fixed issue. Having this PR now referenced to 29400, a reader can now, coming from the core PR easily find this documentation PR as additional info.