docs: correct encoder API documentation

[gregbenz]

What changed

• Document that JPEG output is always available and HEIF/AVIF output is build-dependent.
• List the actual errors returned by uhdr_enc_set_output_format().
• Correct the compressed-input examples and clarify that HEIF/AVIF currently support raw API-0 and API-1 only.

Why

The public header did not match the implementation: it described JPEG as the only output format while also implying compressed HDR and HEIF/AVIF transcoding workflows that are not supported. Accurate API documentation helps callers select valid inputs and handle unsupported formats correctly.

Testing

• Verified the documentation against the encoder setter and JPEG/HEIF/AVIF dispatch paths.
• git diff --check

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