Doc: Lossless JPEG clarifications

- Clarify that lossless JPEG is slower than and doesn't compress as well
  as lossy JPEG.  (That should be obvious, because "lossy" literally
  means that data is thrown away.)
- Re-generate TurboJPEG C API documentation using Doxygen 1.9.8.
- Clarify that setting the data_precision field in jpeg_compress_struct
  to 16 requires lossless mode.
- Explain what the predictor selection value actually does.  (Refer to
  Recommendation ITU-T T.81 (1992) | ISO/IEC 10918-1:1994, Section
  H.1.2.1.)

doc/html/search/variables_8.js

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