docs: count inputs vs elements of input (#5035)

Author: iloveitaly

docs/language/aggregates/count.md

@@ -42,3 +42,12 @@ echo '1 "foo" 10.0.0.1' | zq -z 'yield count()' -
 2(uint64)
 3(uint64)
 ```
+
+Note that the number of input values are counted, unlike the [`len` function](../functions/len.md) which counts the number of elements in a given value:
+```mdtest-command
+echo '[1,2,3]' | zq -z 'count()' -
+```
+=>
+```mdtest-output
+1(uint64)
+```