[DOCS] jpegxl:speed incorrectly listed/explained

[jonnyawsom3]

In the docs jpegxl:speed is described as setting the speed of encoding, even though that's what jpegxl:effort directly above it does. The description even says

Note: in libjxl it named JXL_ENC_FRAME_SETTING_DECODING_SPEED. But it is about encoding speed and compression quality, not decoding speed.

Which is incorrect. The function is meant to adjust decoding speed, but in it's current implementation in libjxl that means disabling expensive coding tools, which also improves encoding speed by chance. In the case of lossless, it can cause exponentially larger filesizes than just lowering the effort setting.