Run `config-to-docs` for `previewJPEGImageDisplayQuality`

[JammingBen]

WHAT Needs to be Documented?

PR owncloud/core#39349 (Add quality setting for JP(E)G preview images) introduces a new config setting previewJPEGImageDisplayQuality to determine the display quality of JPEG previews. This config will be available with OC 10.9.

[mmattel]

This also needs an update in https://doc.owncloud.com/server/10.8/admin_manual/configuration/files/previews_configuration.html