Improve "caption.csv" feature documentation

[sathoro]

The filename required for custom captions is called "caption.csv" (code)

However, there is an error message that refers to it in the plural form as "captions.csv" (code)

It is also referred to as "captions.csv" on the Replicate blog (which is where I originally saw it)

Additionally this page does not mention the caption.csv which I think would be another good place to document it: https://replicate.com/stability-ai/sdxl#training-inputs