Cleanup: move public doc-strings into headers for 'blenkernel'

- Added space below non doc-string comments to make it clear
  these aren't comments for the symbols directly below them.
- Use doxy sections for some headers.
- Minor improvements to doc-strings.

Ref T92709

source/blender/blenkernel/intern/editmesh_tangent.c

@@ -273,13 +273,6 @@ static void emDM_calc_loop_tangents_thread(TaskPool *__restrict UNUSED(pool), vo
   }
 }
 
-/**
- * \see #BKE_mesh_calc_loop_tangent, same logic but used arrays instead of #BMesh data.
- *
- * \note This function is not so normal, its using #BMesh.ldata as input,
- * but output's to #Mesh.ldata.
- * This is done because #CD_TANGENT is cache data used only for drawing.
- */
 void BKE_editmesh_loop_tangent_calc(BMEditMesh *em,
                                     bool calc_active_tangent,
                                     const char (*tangent_names)[MAX_NAME],