Document Collection::median() and Collection::mode() methods

Documents the features added by laravel/framework#14305

I also changed the Collection::avg() documentation with the same code snippet to better demonstrate where they differ since they all are related. Since they can be confusing to understand, I also added reference links to Wikipedia to each one.

Author: paulofreitas

collections.md

@@ -78,8 +78,10 @@ For the remainder of this documentation, we'll discuss each method available on
 [map](#method-map)
 [mapWithKeys](#method-mapwithkeys)
 [max](#method-max)
+[median](#method-median)
 [merge](#method-merge)
 [min](#method-min)
+[mode](#method-mode)
 [nth](#method-nth)
 [only](#method-only)
 [partition](#method-partition)
@@ -147,22 +149,15 @@ The `all` method returns the underlying array represented by the collection:
 <a name="method-avg"></a>
 #### `avg()` {#collection-method}
 
-The `avg` method returns the average of all items in the collection:
+The `avg` method returns the [average value](https://en.wikipedia.org/wiki/Average) of a given key:
 
-    collect([1, 2, 3, 4, 5])->avg();
+    $average = collect([['foo' => 10], ['foo' => 10], ['foo' => 20], ['foo' => 40]])->avg('foo');
 
-    // 3
-
-If the collection contains nested arrays or objects, you should pass a key to use for determining which values to calculate the average:
-
-    $collection = collect([
-        ['name' => 'JavaScript: The Good Parts', 'pages' => 176],
-        ['name' => 'JavaScript: The Definitive Guide', 'pages' => 1096],
-    ]);
+    // 20
 
-    $collection->avg('pages');
+    $average = collect([1, 1, 2, 4])->avg();
 
-    // 636
+    // 2
 
 <a name="method-chunk"></a>
 #### `chunk()` {#collection-method}
@@ -737,6 +732,19 @@ The `max` method returns the maximum value of a given key:
 
     // 5
 
+<a name="method-median"></a>
+#### `median()` {#collection-method}
+
+The `median` method returns the [median value](https://en.wikipedia.org/wiki/Median) of a given key:
+
+    $median = collect([['foo' => 10], ['foo' => 10], ['foo' => 20], ['foo' => 40]])->median('foo');
+
+    // 15
+
+    $median = collect([1, 1, 2, 4])->median();
+
+    // 1.5
+
 <a name="method-merge"></a>
 #### `merge()` {#collection-method}
 
@@ -773,6 +781,19 @@ The `min` method returns the minimum value of a given key:
 
     // 1
 
+<a name="method-mode"></a>
+#### `mode()` {#collection-method}
+
+The `mode` method returns the [mode value](https://en.wikipedia.org/wiki/Mode_(statistics)) of a given key:
+
+    $mode = collect([['foo' => 10], ['foo' => 10], ['foo' => 20], ['foo' => 40]])->mode('foo');
+
+    // [10]
+
+    $mode = collect([1, 1, 2, 4])->mode();
+
+    // [1]
+
 <a name="method-nth"></a>
 #### `nth()` {#collection-method}