man: tc-police.8: Emphasize on the two rate control mechanisms

As Jamal pointed out, there are two different approaches to bandwidth
measurement. Try to make this clear by separating them in synopsis and
also documenting the way to fine-tune avrate.
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

man/man8/tc-police.8

@@ -12,13 +12,21 @@ police - policing action
 .IR BYTES [\fB/ BYTES "] ] ["
 .BI peakrate " RATE"
 ] [
-.BI avrate " RATE"
-] [
 .BI overhead " BYTES"
 ] [
 .BI linklayer " TYPE"
 ] [
-.BI conform-exceed " EXCEEDACT\fR[\fB/\fIEXCEEDACT\fR]"
+.IR CONTROL " ]"
+
+.ti -8
+.BR tc " ... " filter " ... [ " estimator
+.IR "SAMPLE AVERAGE " ]
+.BR "action police avrate"
+.IR RATE " [ " CONTROL " ]"
+
+.ti -8
+.IR CONTROL " :="
+.BI conform-exceed " EXCEEDACT\fR[\fB/\fIEXCEEDACT"
 
 .ti -8
 .IR EXCEEDACT " := { "
@@ -27,7 +35,14 @@ police - policing action
 The
 .B police
 action allows to limit bandwidth of traffic matched by the filter it is
-attached to.
+attached to. Basically there are two different algorithms available to measure
+the packet rate: The first one uses an internal dual token bucket and is
+configured using the
+.BR rate ", " burst ", " mtu ", " peakrate ", " overhead " and " linklayer
+parameters. The second one uses an in-kernel sampling mechanism. It can be
+fine-tuned using the
+.B estimator
+filter parameter.
 .SH OPTIONS
 .TP
 .BI rate " RATE"
@@ -73,6 +88,12 @@ cell sizes, for
 .B ethernet
 no action is taken.
 .TP
+.BI estimator " SAMPLE AVERAGE"
+Fine-tune the in-kernel packet rate estimator.
+.IR SAMPLE " and " AVERAGE
+are time values and control the frequency in which samples are taken and over
+what timespan an average is built.
+.TP
 .BI conform-exceed " EXCEEDACT\fR[\fB/\fIEXCEEDACT\fR]"
 Define how to handle packets which exceed (and, if the second
 .I EXCEEDACT