Document the instant rate.

Hamming up the "angry" meaning just a tad.

content/docs/querying/functions.md

@@ -159,6 +159,31 @@ increase(http_requests_total{job="api-server"}[5m])
 human readability. Use `rate` in recording rules so that increases are tracked
 consistently on a per-second basis.
 
+## `irate()`
+
+`irate(v range-vector)` calculates the per-second instant rate of increase of
+the time series in the range vector. This is based on the last two data points.
+Breaks in monotonicity (such as counter resets due to target restarts) are
+automatically adjusted for.
+
+The following example expression returns the per-second rate of HTTP requests
+looking up to 5 minutes back for the two most recent data points, per time
+series in the range vector:
+
+```
+irate(http_requests_total{job="api-server"}[5m])
+```
+
+`irate` should only be used when graphing volatile, fast-moving counters.
+Use `rate` for alerts and slow-moving counters, as brief changes
+in the rate can reset the `FOR` clause and graphs consisting entirely of rare
+spikes are hard to read.
+
+Note that when combining `irate()` with an aggregation operator (e.g. `sum()`)
+or a function aggregating over time (any function ending in `_over_time`),
+always take a `irate()` first, then aggregate. Otherwise `irate()` cannot detect
+counter resets when your target restarts.
+
 ## `label_replace()`
 
 For each timeseries in `v`, `label_replace(v instant-vector, dst_label string, replacement string,
@@ -216,7 +241,8 @@ over the last 5 minutes, per time series in the range vector:
 rate(http_requests_total{job="api-server"}[5m])
 ```
 
-`rate` should only be used with counters.
+`rate` should only be used with counters. It is best suited for alerting,
+and for graphing of slow-moving counters.
 
 Note that when combining `rate()` with an aggregation operator (e.g. `sum()`)
 or a function aggregating over time (any function ending in `_over_time`),