[DOC] Clara Read Region Documentations

[M-Phuykong]

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It would be helpful to mention the default value CUCIM pads when the user provides a location that is larger than the Image's dimension.

For example: When using CUCIM with an SVS file and I specified the returned region to be larger than the image, it seems like it is padding with the value 255 instead 0. Openslide seems to pad the remaining value with 0 instead.

Describe the documentation you'd like
The padded default value that CUCIM returns if the specified size of the region were to be larger than the image's dimensions or if the location were to be outside the image's dimension coordinate.