Several undocumented namespace constants in XMP_Const.h

### Expected Behaviour

All publicly-declared constants have `\def` and `\brief` comments.

### Actual Behaviour

Several of the constants in XMP_Const.h do not have doc comments explaining their meaning.

Compare (one example each): `kXMP_NS_ASF` (undocumented) vs `kXMP_NS_TIFF` (which has a `\def` and `\brief` description earlier in the file).

Open XMP_Const.h and compare the publicly declared namespace constants, starting with the documented kXMP_NS_TIFF example and the undocumented kXMP_NS_ASF constant. Add def and brief comments explaining each missing constant, then confirm that all publicly declared constants in the file have both documentation tags.

Written by the indexing model from the issue text.