Request to document known limitations and behaviors based on https://issues.redhat.com/browse/RHOSRFE-176 

Description:

Current image_size_total behavior accepts any size of image while the total size is exceeded.
This means that the quota only works for the situation that the image size total is already exceeded.
For example, image_size_total sets 10 MiB. The current behavior accepts an image whose size is larger than 10MiB.
This is not an ideal quota behavior because quota should work as the limit to the size.

The ideal behavior is as follows if image_size_total sets to 10MiB.
1. Image can be uploaded until the total size including all uploaded image is equal or less than image_size_total.
2. If the total size is less than image_size_total, the only accepted image size is (image_size_total - the total size of the current images).