doc/avconv: elaborate on basic functionality.

doc/avconv.texi

@@ -26,6 +26,23 @@ avconv is a very fast video and audio converter that can also grab from
 a live audio/video source. It can also convert between arbitrary sample
 rates and resize video on the fly with a high quality polyphase filter.
 
+avconv reads from an arbitrary number of input "files" (which can be regular
+files, pipes, network streams, grabbing devices, etc.), specified by the
+@code{-i} option, and writes to an arbitrary number of output "files", which are
+specified by a plain output filename. Anything found on the commandline which
+cannot be interpreted as an option is considered to be an output filename.
+
+Each input or output file can in principle contain any number of streams of
+different types (video/audio/subtitle/attachment/data). Allowed number and/or
+types of streams can be limited by the container format. Selecting, which
+streams from which inputs go into output, is done either automatically or with
+the @code{-map} option (see the Stream selection chapter).
+
+To refer to input files in options, you must use their indices (0-based). E.g.
+the first input file is @code{0}, the second is @code{1} etc. Similarly, streams
+within a file are referred to by their indices. E.g. @code{2:3} refers to the
+fourth stream in the third input file. See also the Stream specifiers chapter.
+
 As a general rule, options are applied to the next specified
 file. Therefore, order is important, and you can have the same
 option on the command line multiple times. Each occurrence is
@@ -33,6 +50,10 @@ then applied to the next input or output file.
 Exceptions from this rule are the global options (e.g. verbosity level),
 which should be specified first.
 
+Do not mix input and output files -- first specify all input files, then all
+output files. Also do not mix options which belong to different files. All
+options apply ONLY to the next input or output file and are reset between files.
+
 @itemize
 @item
 To set the video bitrate of the output file to 64kbit/s: