-getting new mail, by adding gnus-group-split-update to
-nnmail-pre-get-new-mail-hook."
+getting new mail, by adding `gnus-group-split-update' to
+`nnmail-pre-get-new-mail-hook'.
+
+A non-nil CATCH-ALL replaces the current value of
+`gnus-group-split-default-catch-all-group'. This variable is only used
+by gnus-group-split-update, and only when its CATCH-ALL argument is
+nil. This argument may contain any fancy split, that will be added as
+the last split in a `|' split produced by `gnus-group-split-fancy',
+unless overridden by any group marked as a catch-all group. Typical
+uses are as simple as the name of a default mail group, but more
+elaborate fancy splits may also be useful to split mail that doesn't
+match any of the group-specified splitting rules. See
+`gnus-group-split-fancy' for details."