Update docs.

docs/Language.md

@@ -549,3 +549,16 @@ grow unbounded as the number of sessions increases. If the programmer knows that
 the `/end/` pattern is the last time a session will be observed, then the datum
 at `$session` will be freed, which keeps `mtail` memory usage under control and
 will improve search time for finding dimensioned metrics.
+
+`del` can be modified with the `after` keyword, signalling that the metric
+should be deleted after some period of no activity.  For example, the
+expression
+
+```
+  del session_start[$session] after 24h
+```
+
+would mean that the datum indexed by `$session` will be removed 24 hours after the last update is recorded.
+
+The del-after form takes any time period supported by the go
+[`time.ParseDuration`](https://golang.org/pkg/time/#ParseDuration) function.

docs/Programming-Guide.md

@@ -201,6 +201,11 @@ the datum referenced by that label from this metric, keeping `mtail`'s memory
 usage under control and speeding up labelset search time (by reducing the
 search space!)
 
+Alternatively, the statement `del connection_time[$pid] after 72h` would do the
+same, but only if `connection_time$pid]` is not changed for 72 hours.  This
+form is more convenient when the connection close event is lossy or difficult
+to determine.
+
 ## Computing moving averages
 
 `mtail` deliberately does not implement complex mathematical functions.  It