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

Ref T92709

source/blender/blendthumb/src/blendthumb.hh

@@ -53,6 +53,10 @@ enum eThumbStatus {
 
 std::optional<blender::Vector<uint8_t>> blendthumb_create_png_data_from_thumb(
     const Thumbnail *thumb);
+/**
+ * This function extracts the thumbnail from the .blend file into thumb.
+ * Returns #BT_OK for success and the relevant error code otherwise.
+ */
 eThumbStatus blendthumb_create_thumb_from_file(struct FileReader *rawfile, Thumbnail *thumb);
 
 /* INTEGER CODES */

source/blender/blendthumb/src/blendthumb_extract.cc

@@ -179,10 +179,6 @@ static eThumbStatus blendthumb_extract_from_file_impl(FileReader *file,
   return BT_INVALID_THUMB;
 }
 
-/**
- * This function extracts the thumbnail from the .blend file into thumb.
- * Returns #BT_OK for success and the relevant error code otherwise.
- */
 eThumbStatus blendthumb_create_thumb_from_file(FileReader *rawfile, Thumbnail *thumb)
 {
   /* Read header in order to identify file type. */