Add prefer and noselect options

chrony.texi

@@ -2387,6 +2387,10 @@ nanosecond).
 @item precision
 Refclock precision (in seconds).  The default is 1e-6 (1 microsecond)
 for SHM refclock, and 1e-9 (1 nanosecond) for SOCK and PPS refclocks.
+@item prefer
+Prefer this source over sources without prefer option.
+@item noselect
+Never select this source.  This is particularly useful for monitoring.
 @end table
 
 @c }}}
@@ -2625,6 +2629,12 @@ selecting that source.  This is useful with low stratum sources that are known
 to be unrealiable or inaccurate and which should be used only when other
 sources are unreachable.
 
+@item prefer
+Prefer this source over sources without prefer option.
+
+@item noselect
+Never select this source.  This is particularly useful for monitoring.
+
 @end table
 @c }}}
 @c {{{ tempcomp