docs: move the riff structure to std-section

If someone intents to document them and the fields we can move them back.

docs/libs/gst-plugins-base-libs-sections.txt

@@ -1042,15 +1042,12 @@ GST_RIFF_y41p
 GST_RIFF_yuy2
 GST_RIFF_yv12
 
-gst_riff_acid
 gst_riff_create_audio_caps
 gst_riff_create_audio_template_caps
 gst_riff_create_iavs_caps
 gst_riff_create_iavs_template_caps
 gst_riff_create_video_caps
 gst_riff_create_video_template_caps
-gst_riff_dmlh
-gst_riff_index_entry
 gst_riff_init
 gst_riff_parse_chunk
 gst_riff_parse_file_header
@@ -1060,11 +1057,14 @@ gst_riff_parse_strf_iavs
 gst_riff_parse_strf_vids
 gst_riff_parse_strh
 gst_riff_read_chunk
+<SUBSECTION Standard>
+gst_riff_acid
+gst_riff_dmlh
+gst_riff_index_entry
 gst_riff_strf_auds
 gst_riff_strf_iavs
 gst_riff_strf_vids
 gst_riff_strh
-<SUBSECTION Standard>
 </SECTION>