doc/libavcodec: improve wording in description

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/libavcodec.texi

@@ -12,10 +12,14 @@
 @chapter Description
 @c man begin DESCRIPTION
 
-libavcodec is a library providing a generic encoding/decoding
-framework, and containing decoders and encoders for audio, video and
+The libavcodec library provides a generic encoding/decoding framework
+and contains multiple decoders and encoders for audio, video and
 subtitle streams.
 
+The shared architecture provides various services ranging from bit
+stream I/O to DSP optimizations, and makes it suitable for
+implementing robust and fast codecs as well as for experimentation.
+
 @c man end DESCRIPTION
 
 @chapter Codec Options