doc: Libavfilter English cleanup

This aims to standardize the English used in the libavfilter
documentation, describing the same thing in the same way more often,
and significantly reduce the amount of outright errors, including
run-on sentences and sentence fragments.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

doc/libavfilter.texi

@@ -11,10 +11,10 @@
 
 @chapter Introduction
 
-Libavfilter is the filtering API of Libav. It is the substitute of the
-now deprecated 'vhooks' and started as a Google Summer of Code project.
+Libavfilter is the filtering API of Libav. It replaces 'vhooks', and
+started as a Google Summer of Code project.
 
-But note that there may still be serious bugs in the code and its API
+Note that there may still be serious bugs in the code and its API
 and ABI should not be considered stable yet!
 
 @chapter Tutorial
@@ -31,33 +31,33 @@ input --> split --> fifo -----------------------> overlay --> output
             +------> fifo --> crop --> vflip --------+
 @end example
 
-splits the stream in two streams, sends one stream through the crop filter
-and the vflip filter before merging it back with the other stream by
+splits the stream in two streams, then sends one stream through the crop filter
+and the vflip filter, before merging it back with the other stream by
 overlaying it on top. You can use the following command to achieve this:
 
 @example
 ./avconv -i input -vf "[in] split [T1], fifo, [T2] overlay=0:H/2 [out]; [T1] fifo, crop=iw:ih/2:0:ih/2, vflip [T2]" output
 @end example
 
-The result will be that in output the top half of the video is mirrored
-onto the bottom half.
+The result will be that the top half of the video is mirrored
+onto the bottom half of the output video.
 
 Video filters are loaded using the @var{-vf} option passed to
 avconv or to avplay. Filters in the same linear chain are separated by
-commas. In our example, @var{split, fifo, overlay} are in one linear
-chain, and @var{fifo, crop, vflip} are in another. The points where
-the linear chains join are labeled by names enclosed in square
-brackets. In our example, that is @var{[T1]} and @var{[T2]}. The magic
+commas. In our example, @var{split}, @var{fifo}, and @var{overlay} are in one
+linear chain, and @var{fifo}, @var{crop}, and @var{vflip} are in another. The
+points where the linear chains join are labeled by names enclosed in square
+brackets. In our example, they join at @var{[T1]} and @var{[T2]}. The magic
 labels @var{[in]} and @var{[out]} are the points where video is input
 and output.
 
-Some filters take in input a list of parameters: they are specified
-after the filter name and an equal sign, and are separated each other
+Some filters take a list of parameters: they are specified
+after the filter name and an equal sign, and are separated
 by a semicolon.
 
-There exist so-called @var{source filters} that do not have a video
-input, and we expect in the future some @var{sink filters} that will
-not have video output.
+There are so-called @var{source filters} that do not take video
+input, and we expect that some @var{sink filters} will
+not have video output, at some point in the future.
 
 @chapter graph2dot