Clarify 'age' in docs

Change-Id: Ie20b9b21aaa2d3d7f44ec751da87a2638e77d592

Documentation/user-search.txt

@@ -53,7 +53,7 @@ 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`:
+to include a unit suffix, for example `-age:2d`:
 +
 * s, sec, second, seconds
 * m, min, minute, minutes
@@ -63,6 +63,10 @@ to include a unit suffix, for example `age:2d`:
 * mon, month, months (`1 month` is treated as `30 days`)
 * y, year, years (`1 year` is treated as `365 days`)
 
+`age` can be used both forward and backward looking: `age:2d`
+means 'everything older than 2 days' while `-age:2d` means
+'everything with an age of at most 2 days'.
+
 [[assignee]]
 assignee:'USER'::
 +