Make documentation for absent() not, uhm, absent

content/docs/querying/functions.md

@@ -10,6 +10,24 @@ sort_rank: 3
 `abs(v vector)` returns the input vector with all sample values converted to
 their absolute value.
 
+## `absent(v vector)`
+
+`absent(v vector)` returns an empty vector if the vector passed to it has any
+elements and a 1-element vector with the value 1 if the vector passed to it has
+no elements.
+
+In the second case, `absent()` tries to be smart about deriving labels of
+the 1-element output vector from the input vector:
+
+| Expression                                       | Result
+|--------------------------------------------------|--------
+| absent(nonexistent{job="myjob"})                 | {job="myjob"}
+| absent(nonexistent{job="myjob",instance=~".*"})  | {job="myjob"}
+| absent(sum(nonexistent{job="myjob"}))            | {}
+
+This is useful for alerting on when no time series
+exist for a given metric name and label combination.
+
 ## `count_scalar()`
 
 `count_scalar(v instant-vector)` returns the number of elements in a time series