doc/bitstream-filters.texi: add documentation for the aac_adtstoasc filter

Based on the notes in libavcodec/aac_adtstoasc_bsf.c.

doc/bitstream_filters.texi

@@ -17,6 +17,15 @@ Below is a description of the currently available bitstream filters.
 
 @section aac_adtstoasc
 
+Convert MPEG-2/4 AAC ADTS to MPEG-4 Audio Specific Configuration
+bitstream filter.
+
+This filter creates an MPEG-4 AudioSpecificConfig from an MPEG-2/4
+ADTS header and removes the ADTS header.
+
+This is required for example when copying an AAC stream from a raw
+ADTS AAC container to a FLV or a MOV/MP4 file.
+
 @section chomp
 
 @section dump_extradata