Corrected JPEG subsampling documentation

[radarhere]

Alternative to #4570

https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#jpeg

subsampling
If present, sets the subsampling for the encoder.

• keep: Only valid for JPEG files, will retain the original image setting.
• 4:4:4, 4:2:2, 4:2:0: Specific sampling values
• -1: equivalent to keep

Discussion in that PR revealed that -1 is not always equivalent to keep. The PR suggested fixing that discrepancy by changing Pillow to make -1 equivalent to keep.

This PR instead suggests to leave Pillow behaviour unchanged, and just correct the documentation.

In Python, -1 is just the default value to pass to C.
In C, -1 is actually not a setting at all, it's just the default behaviour.
So I suggest removing -1 from the documentation. If the user wants the default behaviour, they can just not specify the subsampling option. -1 is also the default for "quality", and that isn't mentioned in the documentation.

I'm also explicitly mentioning that if subsampling is not specified, then it will be determined by the JPEG dependency.

[QSchulz]

🎉 thanks for taking care of that!

[hugovk]

Thank you both!