Add operators for absolute last-updated-on search

Some users may have been using sortkey to exclude changes older than
a certain date. This is possible but inconvenient to mimic with an
age: query, so provide operators for absolute searches.

The semantics of these operators are similar to those of `git log`,
except for some incompatibilities/quirks of JGit's GitDateParser.

Change-Id: I52fee758052a5644beb3c97bf727f92129245f3f
diff --git a/Documentation/user-search.txt b/Documentation/user-search.txt
index 0c6abb8..41426fb 100644
--- a/Documentation/user-search.txt
+++ b/Documentation/user-search.txt
@@ -64,6 +64,20 @@
 * mon, month, months (`1 month` is treated as `30 days`)
 * y, year, years (`1 year` is treated as `365 days`)
 
+[[before_until]]
+before:'TIME'/until:'TIME'
++
+Changes modified before the given 'TIME', inclusive. With no time,
+assumes 00:00:00 in the local time zone. Supports many formats supported
+by `git log`.
+
+[[after_since]]
+after:'TIME'/since:'TIME'
++
+Changes modified before the given 'TIME', inclusive. With no time,
+assumes 00:00:00 in the local time zone. Supports many formats supported
+by `git log`.
+
 [[change]]
 change:'ID'::
 +