doc: improve description of refclock filter option

diff --git a/doc/chrony.conf.adoc b/doc/chrony.conf.adoc
index a21489800fb3b78e1c65b8b06375866a3602d540..f4dff3311ef80e2ad1509c7f5f16d795abc68e5b 100644 (file)
--- a/doc/chrony.conf.adoc
+++ b/doc/chrony.conf.adoc
@@ -642,7 +642,9 @@ noise in the measurements. With each poll about 40 percent of the stored
 samples are discarded and one final sample is calculated as an average of the
 remaining samples. If the length is 4 or more, at least 4 samples have to be
 collected between polls. For lengths below 4, the filter has to be full. The
-default is 64.
+default is 64. With drivers that perform their own polling (PPS, PHC, SHM), the
+maximum value is adjusted to the number of driver polls per source poll, i.e.
+2^(_poll_ - _dpoll_).
 *prefer*:::
 Prefer this source over sources without the prefer option.
 *noselect*:::