Use code style in documentation for referred functions.

Author: louisdh

stdlib/public/core/BidirectionalCollection.swift

@@ -205,10 +205,10 @@ extension BidirectionalCollection where SubSequence == BidirectionalSlice<Self>
 extension BidirectionalCollection where SubSequence == Self {
   /// Removes and returns the last element of the collection.
   /// 
-  /// popLast() should generally be preferred over removeLast() 
+  /// `popLast()` should generally be preferred over `removeLast()` 
   /// since it is safer to use. 
   /// 
-  /// You should only opt for removeLast() if you are certain the collection 
+  /// You should only opt for `removeLast()` if you are certain the collection 
   /// is not empty and performance is an issue.
   ///
   /// - Returns: The last element of the collection if the collection has one
@@ -228,7 +228,7 @@ extension BidirectionalCollection where SubSequence == Self {
   /// You should only use this if you are certain the collection 
   /// is not empty and performance is an issue.
   /// 
-  /// Otherwise opt for popLast() since it is safer.
+  /// Otherwise opt for `popLast()` since it is safer.
   ///
   /// - Returns: The last element of the collection.
   ///