Document offset modifier.

content/docs/querying/basics.md

@@ -107,6 +107,30 @@ a `job` label set to `prometheus`:
 
     http_requests_total{job="prometheus"}[5m]
 
+### Offset modifier
+
+The `offset` modifier allows changing the time offset for individual
+instant and range vectors in a query.
+
+For example, the following expression returns the value of `foo` 5
+minutes in the past relative to the current query evaluation time:
+
+    foo offset 5m
+
+Note that the `offset` modifier always needs to follow the selector
+immediately, i.e. the following would be correct:
+
+    sum(foo offset 5m) // GOOD.
+
+While the following would be *incorrect*:
+
+    sum(foo) offset 5m // INVALID.
+
+The same works for range vectors. This returns the 5-minutes rate that
+`foo` had a week ago:
+
+    rate(foo[5m] offset 1w)
+    
 ## Operators
 
 Prometheus supports many binary and aggregation operators. These are described