doc(extension method): add example with braces

and remove note about type parameters for collective extensions.
Since we have a unified syntax it should be clear that the same we
explained for extension methods also apply to collective extensions
as they are the same thing.

Author: robstoll

docs/docs/reference/contextual/extension-methods.md

@@ -113,6 +113,18 @@ extension (ss: Seq[String]):
   def longestString: String = longestStrings.head
 ```
 
+The same can be written with braces as follows (note that indented regions can still be used inside braces):
+```scala
+extension (ss: Seq[String]) {
+
+  def longestStrings: Seq[String] =
+    val maxLength = ss.map(_.length).max
+    ss.filter(_.length == maxLength)
+
+  def longestString: String = longestStrings.head
+}
+```
+
 Note the right-hand side of `longestString`: it calls `longestStrings` directly, implicitly
 assuming the common extended value `ss` as receiver.
 
@@ -129,16 +141,6 @@ extension (ss: Seq[String])
   def longestString: String = ss.longestStrings.head
 ```
 
-Collective extensions also can take type parameters and have using clauses. Example
-
-```scala
-extension [T](xs: List[T])(using Ordering[T]):
-  def smallest(n: Int): List[T] = xs.sorted.take(n)
-  def smallestIndices(n: Int): List[Int] =
-    val limit = smallest(n).max
-    xs.zipWithIndex.collect { case (x, i) if x <= limit => i }
-```
-
 ### Translation of Calls to Extension Methods
 
 To convert a reference to an extension method, the compiler has to know about the extension