Clarify partitioningIndex(where:) behavior in inline documentation

The returned index isn’t necessarily an index of an element. In the case where it returns `endIndex`, it’s not valid for subscripting the collection.

Resolves #170

Author: mdznr

Sources/Algorithms/Partition.swift

@@ -168,10 +168,9 @@ extension MutableCollection where Self: BidirectionalCollection {
 //===----------------------------------------------------------------------===//
 
 extension Collection {
-  /// Returns the index of the first element in the collection that matches
-  /// the predicate.
+  /// Returns the start index of the second partition of a collection already
+  /// partitioned according to the predicate.
   ///
-  /// The collection must already be partitioned according to the predicate.
   /// That is, there should be an index `i` where for every element in
   /// `collection[..<i]` the predicate is `false`, and for every element in
   /// `collection[i...]` the predicate is `true`.