Reword the explanation.

docs/pv.1

@@ -382,10 +382,11 @@ exhausted, read all of \fIFILE\fR and write it to the output.
 \*(lq\fB-\fR\*(rq, in which case \fBpv\fR creates a temporary file for this
 purpose, and automatically removes it afterwards.
 .IP
-This can be useful if you have a pipeline which generates data (your
-input) quickly but you don't know the size, and you wish to pass it to some
-slower process, once all of the input has been generated and you know its
-size, so you can see its progress.
+A common use for this is this where one part of a pipeline generates data
+quickly but of unknown size, and the next part processes it slowly.
+With \*(lq\fIquick\fR\~\fB|\~pv\~\-U\~\-\~|\fR\~\fIslow\fR\*(rq, a progress
+bar is displayed with unknown size for the \fIquick\fR part, then a second
+is shown with a known size for the \fIslow\fR part.
 .\"
 .\"
 .SS "Alternative operating modes"

docs/pv.1.md

@@ -395,10 +395,11 @@ are explicitly switched on will be shown.
     **pv** creates a temporary file for this purpose, and automatically
     removes it afterwards.
 
-    This can be useful if you have a pipeline which generates data (your
-    input) quickly but you don\'t know the size, and you wish to pass it
-    to some slower process, once all of the input has been generated and
-    you know its size, so you can see its progress.
+    A common use for this is this where one part of a pipeline generates
+    data quickly but of unknown size, and the next part processes it
+    slowly. With "*quick* **\| pv -U - \|** *slow*", a progress bar is
+    displayed with unknown size for the *quick* part, then a second is
+    shown with a known size for the *slow* part.
 
 ## Alternative operating modes