Doc: Improve doc about transformers

README.md

@@ -135,16 +135,18 @@ The `EtlState` object contains all elements relative to the state of your ETL wo
 Difference between `yield` and `return` in transformers
 ------------------------------------------------------
 
+A transformer can either return a value, or yield values (like the example above).
+
 The `EtlExecutor::transformWith()` method accepts an unlimited number of transformers as arguments.
 
 When you chain transformers, keep in mind that every transformer will get:
 - Either the returned value passed from the previous transformer
-- Either an array of every yielded value from the previous transformer
+- Either a `Generator` of every yielded value from the previous transformer
 
-But the last transformer of the chain (or your only one transformer) is deterministic to know what will be passed to the loader (either a return  value, or a generator):
-- If your transformer `returns` a value, this value will be passed to the loader.
-- If your transformer `returns` an array of values (or whatever iterable), that return value will be passed to the loader.
-- If your transformer `yields` values, each yielded value will be passed to the loader.
+But the last transformer of the chain (or your only one transformer) is deterministic to know what will be passed to the loader:
+- If your transformer `returns` a value, this value will be passed to the loader (and the loader will be called once for this value).
+- If your transformer `returns` an array of values (or whatever iterable), that return value will be passed to the loader (and the loader will be called once for this value).
+- If your transformer `yields` values, each yielded value will be passed to the loader (and the loader will be called for each yielded value).
 
 Using events
 ------------