doc: Clarify the avconv section about -re

Signed-off-by: Martin Storsjö <martin@martin.st>

doc/avconv.texi

@@ -719,7 +719,10 @@ Dump each input packet to stderr.
 @item -hex (@emph{global})
 When dumping packets, also dump the payload.
 @item -re (@emph{input})
-Read input at native frame rate. Mainly used to simulate a grab device.
+Read input at native frame rate. Mainly used to simulate a grab device
+or live input stream (e.g. when reading from a file). Should not be used
+with actual grab devices or live input streams (where it can cause packet
+loss).
 @item -vsync @var{parameter}
 Video sync method.