Add age: operator to change queries

The age operator looks for changes that have not been modified
within the argument.  E.g. "age:3d" will find changes that haven't
been touched in the last 3 days.

This supports searching for stale changes that maybe need to be
considered for further action.

Change-Id: I35c843099aac7bb795613c5898e84ef0a60f91cc
Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/Documentation/user-search.txt b/Documentation/user-search.txt
index 30d26f0..40b9a67 100644
--- a/Documentation/user-search.txt
+++ b/Documentation/user-search.txt
@@ -48,6 +48,21 @@
 are added to the same query string, they further restrict the
 returned results.
 
+[[age]]
+age:'AGE'::
++
+Amount of time that has expired since the change was last updated
+with a review comment or new patch set.  The age must be specified
+to include a unit suffix, for example `age:2d`:
++
+* s, sec, second, seconds
+* m, min, minute, minutes
+* h, hr, hour, hours
+* d, day, days
+* w, week, weeks (`1 week` is treated as `7 days`)
+* mon, month, months (`1 month` is treated as `30 days`)
+* y, year, years (`1 year` is treated as `365 days`)
+
 [[change]]
 change:'ID'::
 +