Mention that the --query PID needs to be owned by the same user (#101).

docs/pv.1

@@ -400,7 +400,8 @@ Note that some options cannot be changed while running, such as
 .TP
 .BI \-Q\  PID \fR,\ \fB\-\-query\  PID
 Display the transfer progress of another instance of \fBpv\fR with process
-ID \fIPID\fR.
+ID \fIPID\fR, owned by the same user.
+.IP
 Some output modifiers can only be used while querying another process if
 they match those of that process - such as
 \*(lq\fB\-\-line\-mode\fR\*(rq,

docs/pv.1.md

@@ -415,11 +415,12 @@ are explicitly switched on will be shown.
 **-Q PID, \--query PID**
 
 :   Display the transfer progress of another instance of **pv** with
-    process ID *PID*. Some output modifiers can only be used while
-    querying another process if they match those of that process - such
-    as "**\--line-mode**", "**\--null**", and
-    "**\--average-rate-window**". Data transfer modifiers will have no
-    effect.
+    process ID *PID*, owned by the same user.
+
+    Some output modifiers can only be used while querying another
+    process if they match those of that process - such as
+    "**\--line-mode**", "**\--null**", and "**\--average-rate-window**".
+    Data transfer modifiers will have no effect.
 
 ## Other options