doc: document the "duration_longer" filter

Author: MaxKellermann

doc/index.rst

@@ -283,6 +283,10 @@ The following filters are available:
 - :samp:`date=YYYY-MM-DD` is a shortcut which shows records on a
   certain date (according to the client's time zone)
 - :samp:`today` is a shortcut which shows records only of today
+- :samp:`duration_longer=DURATION` shows only records with a duration
+  longer than the specified value.  The value is a positive integer
+  with one of the units ``us``, ``ms``, ``s``, ``m``, ``h``, ``d``.
+  Example: :samp:`duration_longer=500ms`.
 - :samp:`status=STATUS[:END]` shows only records with the specified
   status.  If "END" is also given, then this is the open end of a
   range.  Example: :samp:`status=500:600` shows all server errors.

src/client/Main.cxx

@@ -619,6 +619,7 @@ try {
 			   "    [uri-prefix=VALUE]\n"
 			   "    [generator=VALUE]\n"
 			   "    [since=ISO8601] [until=ISO8601] [date=YYYY-MM-DD] [today]\n"
+			   "    [duration_longer=DURATION]\n"
 			   "    [window=COUNT[@SKIP]]\n"
 			   "  stats\n"
 			   "  inject <RAWFILE\n"