Truncate files written with "-o".

docs/pv.1

@@ -348,7 +348,7 @@ invocations in a single, long, pipeline.
 Write data to
 .I FILE
 rather than standard output.  If the file already exists, it will be
-overwritten from the start, but not truncated.
+truncated.
 .TP
 .BI \-L\  RATE \fR,\ \fB\-\-rate-limit\  RATE
 Limit the transfer to a maximum of

docs/pv.1.md

@@ -260,8 +260,7 @@ that are explicitly switched on will be shown.
 **-o FILE, \--output FILE**
 
 :   Write data to *FILE* rather than standard output. If the file
-    already exists, it will be overwritten from the start, but not
-    truncated.
+    already exists, it will be truncated.
 
 **-L RATE, \--rate-limit RATE**