doc/ffmpeg: document more deeply -re option.

Signed-off-by: Clément Bœsch <ubitux@gmail.com>
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/ffmpeg.texi

@@ -820,6 +820,13 @@ Dump each input packet to stderr.
 When dumping packets, also dump the payload.
 @item -re (@emph{input})
 Read input at native frame rate. Mainly used to simulate a grab device.
+By default @command{ffmpeg} attempts to read the input(s) as fast as possible.
+This option will slow down the reading of the input(s) to the native frame rate
+of the input(s). It is useful for real-time output (e.g. live streaming). If
+your input(s) is coming from some other live streaming source (through HTTP or
+UDP for example) the server might already be in real-time, thus the option will
+likely not be required. On the other hand, this is meaningful if your input(s)
+is a file you are trying to push in real-time.
 @item -loop_input
 Loop over the input stream. Currently it works only for image
 streams. This option is used for automatic FFserver testing.