docs: add description for toKeySet operator (#257)

Co-authored-by: Josep M Sobrepere <jm.sobrepere@gmail.com>

Author: voliva

packages/utils/src/partitionByKey.ts

@@ -23,7 +23,7 @@ export interface KeyChanges<K> {
  * @param streamSelector Function to apply to each resulting group
  * @returns [1, 2]
  * 1. A function that accepts a key and returns the stream for the group of that key.
- * 2. A stream with the list of active keys
+ * 2. A stream of KeyChanges, an object that describes what keys have been added or deleted.
  */
 export function partitionByKey<T, K, R>(
   stream: Observable<T>,
@@ -39,7 +39,7 @@ export function partitionByKey<T, K, R>(
  * @param keySelector Function that specifies the key for each element in `stream`
  * @returns [1, 2]
  * 1. A function that accepts a key and returns the stream for the group of that key.
- * 2. A stream with the list of active keys
+ * 2. A stream of KeyChanges, an object that describes what keys have been added or deleted.
  */
 export function partitionByKey<T, K>(
   stream: Observable<T>,

packages/utils/src/toKeySet.ts

@@ -1,6 +1,10 @@
 import { Observable, OperatorFunction } from "rxjs"
 import { KeyChanges } from "./partitionByKey"
 
+/**
+ * Operator function that maps a stream of KeyChanges into a Set that contains
+ * the active keys.
+ */
 export function toKeySet<K>(): OperatorFunction<KeyChanges<K>, Set<K>> {
   return (source$) =>
     new Observable<Set<K>>((observer) => {