Use @verbatim instead of @example for ASCII arts

Partially fixes #3869.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg.texi

@@ -80,7 +80,7 @@ The format option may be needed for raw input files.
 The transcoding process in @command{ffmpeg} for each output can be described by
 the following diagram:
 
-@example
+@verbatim
  _______              ______________
 |       |            |              |
 | input |  demuxer   | encoded data |   decoder
@@ -99,7 +99,7 @@ the following diagram:
 |________|           |______________|
 
 
-@end example
+@end verbatim
 
 @command{ffmpeg} calls the libavformat library (containing demuxers) to read
 input files and get packets containing encoded data from them. When there are
@@ -124,7 +124,7 @@ Simple filtergraphs are those that have exactly one input and output, both of
 the same type. In the above diagram they can be represented by simply inserting
 an additional step between decoding and encoding:
 
-@example
+@verbatim
  _________                        ______________
 |         |                      |              |
 | decoded |                      | encoded data |
@@ -136,19 +136,19 @@ an additional step between decoding and encoding:
                 | frames   |
                 |__________|
 
-@end example
+@end verbatim
 
 Simple filtergraphs are configured with the per-stream @option{-filter} option
 (with @option{-vf} and @option{-af} aliases for video and audio respectively).
 A simple filtergraph for video can look for example like this:
 
-@example
+@verbatim
  _______        _____________        _______        ________
 |       |      |             |      |       |      |        |
 | input | ---> | deinterlace | ---> | scale | ---> | output |
 |_______|      |_____________|      |_______|      |________|
 
-@end example
+@end verbatim
 
 Note that some filters change frame properties but not frame contents. E.g. the
 @code{fps} filter in the example above changes number of frames, but does not
@@ -161,7 +161,7 @@ processing chain applied to one stream. This is the case, for example, when the
 more than one input and/or output, or when output stream type is different from
 input. They can be represented with the following diagram:
 
-@example
+@verbatim
  _________
 |         |
 | input 0 |\                    __________
@@ -179,7 +179,7 @@ input. They can be represented with the following diagram:
 | input 2 |/
 |_________|
 
-@end example
+@end verbatim
 
 Complex filtergraphs are configured with the @option{-filter_complex} option.
 Note that this option is global, since a complex filtergraph, by its nature,
@@ -198,14 +198,14 @@ step for the specified stream, so it does only demuxing and muxing. It is useful
 for changing the container format or modifying container-level metadata. The
 diagram above will, in this case, simplify to this:
 
-@example
+@verbatim
  _______              ______________            ________
 |       |            |              |          |        |
 | input |  demuxer   | encoded data |  muxer   | output |
 | file  | ---------> | packets      | -------> | file   |
 |_______|            |______________|          |________|
 
-@end example
+@end verbatim
 
 Since there is no decoding or encoding, it is very fast and there is no quality
 loss. However, it might not work in some cases because of many factors. Applying

doc/ffserver.texi

@@ -118,7 +118,8 @@ Multiple streams can be connected to the same feed.
 
 For example, you can have a situation described by the following
 graph:
-@example
+
+@verbatim
                _________       __________
               |         |     |          |
 ffmpeg 1 -----| feed 1  |-----| stream 1 |
@@ -143,7 +144,8 @@ ffmpeg 2 -----| feed 3  |-----| stream 4 |
               |         |     |          |
               | file 1  |-----| stream 5 |
               |_________|     |__________|
-@end example
+
+@end verbatim
 
 @anchor{FFM}
 @section FFM, FFM2 formats

doc/filters.texi

@@ -8,13 +8,13 @@ outputs.
 To illustrate the sorts of things that are possible, we consider the
 following filtergraph.
 
-@example
+@verbatim
                 [main]
 input --> split ---------------------> overlay --> output
             |                             ^
             |[tmp]                  [flip]|
             +-----> crop --> vflip -------+
-@end example
+@end verbatim
 
 This filtergraph splits the input stream in two streams, then sends one
 stream through the crop filter and the vflip filter, before merging it