+registry will keep. If the registry has reached or exceeded this
+size, it will reject insertion of new entries.
+@end defvar
+
+@defvar gnus-registry-prune-factor
+This option (a float between 0 and 1) controls how much the registry
+is cut back during pruning. In order to prevent constant pruning, the
+registry will be pruned back to less than
+@code{gnus-registry-max-entries}. This option controls exactly how
+much less: the target is calculated as the maximum number of entries
+minus the maximum number times this factor. The default is 0.1:
+i.e. if your registry is limited to 50000 entries, pruning will try to
+cut back to 45000 entries. Entries with keys marked as precious will
+not be pruned.