Add docs for RegisteredDimensionExtractionFn

docs/content/querying/dimensionspecs.md

@@ -308,6 +308,26 @@ This allows distinguishing between a null dimension and a lookup resulting in a
 For example, specifying `{"":"bar","bat":"baz"}` with dimension values `[null, "foo", "bat"]` and replacing missing values with `"oof"` will yield results of `["bar", "oof", "baz"]`.
 Omitting the empty string key will cause the missing value to take over. For example, specifying `{"bat":"baz"}` with dimension values `[null, "foo", "bat"]` and replacing missing values with `"oof"` will yield results of `["oof", "oof", "baz"]`.
 
+### Registered Lookup Extraction Function
+
+While it is recommended that the [lookup dimension spec](#lookup-dimensionspecs) be used whenever possible, any lookup that is registered for use as a lookup dimension spec can be used as a dimension extraction.
+
+The specification for dimension extraction using dimension specification named lookups is formatted as per the following example:
+
+```json
+{
+  "type":"registeredLookup",
+  "lookup":"some_lookup_name",
+  "retainMissingValue":true,
+  "injective":false
+}
+```
+
+All the flags for [lookup extraction function](#lookup-extraction-function) apply here as well.
+
+In general, the dimension specification should be used. This dimension **extraction** implementation is made available for testing, validation, and transitioning from dimension extraction to the dimension specification style lookups.
+There is also a chance that a feature uses dimension extraction in such a way that it is not applied to dimension specification lookups. Such a scenario should be brought to the attention of the development mailing list.
+
 ### Cascade Extraction Function
 
 Provides chained execution of extraction functions.