wording change to the simple description in the note

spec.emu

@@ -25,7 +25,7 @@ location: https://tc39.es/proposal-array-grouping/
   <emu-clause id="sec-array.prototype.groupby">
     <h1>Array.prototype.groupBy ( _callbackfn_ [ , _thisArg_ ] )</h1>
     <emu-note>
-      <p>_callbackfn_ should be a function that accepts three arguments and returns a value that is coercible to a property. `groupBy` calls _callbackfn_ once for each element in the array, in ascending order, and constructs a new object of arrays, where the property key is the return value and array is filled with the items that return that property key.</p>
+      <p>_callbackfn_ should be a function that accepts three arguments. `groupBy` calls _callbackfn_ once for each element in the array, in ascending order, and constructs a new object of arrays. Each value returned by _callbackfn_ is coerced to a property key, and the associated element is included in the array in the constructed object according to this property key.</p>
       <p>If a _thisArg_ parameter is provided, it will be used as the *this* value for each invocation of _callbackfn_. If it is not provided, *undefined* is used instead.</p>
       <p>_callbackfn_ is called with three arguments: the value of the element, the index of the element, and the object being traversed.</p>
       <p>`groupBy` does not directly mutate the object on which it is called but the object may be mutated by the calls to _callbackfn_.</p>
@@ -61,7 +61,7 @@ location: https://tc39.es/proposal-array-grouping/
   <emu-clause id="sec-array.prototype.groupbymap">
     <h1>Array.prototype.groupByMap ( _callbackfn_ [ , _thisArg_ ] )</h1>
     <emu-note>
-      <p>_callbackfn_ should be a function that accepts three arguments and returns any value. `groupByMap` calls _callbackfn_ once for each element in the array, in ascending order, and constructs a new Map of arrays, where the key is the return value and array is filled with the items that return that key.</p>
+      <p>_callbackfn_ should be a function that accepts three arguments. `groupByMap` calls _callbackfn_ once for each element in the array, in ascending order, and constructs a new Map of arrays. Each value returned by _callbackfn_ is used as a key in the Map, and the associated element is included in the array in the constructed Map according to this key.</p>
       <p>If a _thisArg_ parameter is provided, it will be used as the *this* value for each invocation of _callbackfn_. If it is not provided, *undefined* is used instead.</p>
       <p>_callbackfn_ is called with three arguments: the value of the element, the index of the element, and the object being traversed.</p>
       <p>`groupByMap` does not directly mutate the object on which it is called but the object may be mutated by the calls to _callbackfn_.</p>