doc: apply various grammar fixes

The first sentence of each of the modified man pages are worded a bit
awkwardly. These minor copy-edits should make them clearer.
Signed-off-by: Bryce Harrington <b.harrington@samsung.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/demuxers.texi

 @chapter Demuxers
 @c man begin DEMUXERS
 
-Demuxers are configured elements in FFmpeg which allow to read the
+Demuxers are configured elements in FFmpeg that can read the
 multimedia streams from a particular type of file.
 
 When you configure your FFmpeg build, all the supported demuxers

doc/ffmpeg.texi

@@ -1137,7 +1137,7 @@ then it will search for the file @file{libvpx-1080p.ffpreset}.
 
 @itemize
 @item
-For streaming at very low bitrate application, use a low frame rate
+For streaming at very low bitrates, use a low frame rate
 and a small GOP size. This is especially true for RealVideo where
 the Linux player does not seem to be very fast, so it can miss
 frames. An example is:

doc/outdevs.texi

 @chapter Output Devices
 @c man begin OUTPUT DEVICES
 
-Output devices are configured elements in FFmpeg which allow to write
+Output devices are configured elements in FFmpeg that can write
 multimedia data to an output device attached to your system.
 
 When you configure your FFmpeg build, all the supported output devices

doc/protocols.texi

 @chapter Protocols
 @c man begin PROTOCOLS
 
-Protocols are configured elements in FFmpeg which allow to access
-resources which require the use of a particular protocol.
+Protocols are configured elements in FFmpeg that enable access to
+resources that require specific protocols.
 
 When you configure your FFmpeg build, all the supported protocols are
 enabled by default. You can list all available ones using the