[stdlib] Add missing abstracts to a few types

Author: natecook1000

stdlib/public/core/Collection.swift

@@ -152,6 +152,11 @@ public protocol IndexableBase {
   func formIndex(after i: inout Index)
 }
 
+/// A type that provides subscript access to its elements, with forward index
+/// traversal.
+///
+/// In most cases, it's best to ignore this protocol and use the `Collection`
+/// protocol instead, because it has a more complete interface.
 public protocol Indexable : IndexableBase {
   /// A type that can represent the number of steps between a pair of
   /// indices.

stdlib/public/core/FloatingPoint.swift.gyb

@@ -488,11 +488,13 @@ public protocol FloatingPoint: Comparable, IntegerLiteralConvertible,
   var isCanonical: Bool { get }
 }
 
+/// The sign of a floating-point value.
 public enum FloatingPointSign: Int {
   case plus
   case minus
 }
 
+/// The IEEE 754 floating-point classes.
 public enum FloatingPointClassification {
   case signalingNaN
   case quietNaN

stdlib/public/core/Index.swift

@@ -10,6 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+/// This protocol is an implementation detail of `Integer`; do not use it
+/// directly.
 @_show_in_interface
 public protocol _Incrementable : Equatable {}
 

stdlib/public/core/Indices.swift.gyb

@@ -14,17 +14,20 @@
 from gyb_stdlib_support import (
     TRAVERSALS,
     collectionForTraversal,
-    defaultIndicesForTraversal
+    defaultIndicesForTraversal,
+    documentationNameForTraversal
 )
 }%
 
 % for Traversal in TRAVERSALS:
 %   Self = defaultIndicesForTraversal(Traversal)
+%   collection = documentationNameForTraversal(Traversal)
 
 // FIXME(ABI)(compiler limitation): There should be just one default
 // indices type that has conditional conformances to
 // `BidirectionalCollection` and `RandomAccessCollection`.
 
+/// A collection of indices for an arbitrary ${collection}.
 public struct ${Self}<
   Elements : ${collectionForTraversal(Traversal).replace('Collection', 'Indexable')}
   // FIXME(ABI)(compiler limitation):

utils/gyb_stdlib_support.py

@@ -47,3 +47,13 @@ def defaultIndicesForTraversal(traversal):  # noqa (N802)
     if traversal == 'RandomAccess':
         return 'DefaultRandomAccessIndices'
     assert False, 'unknown traversal'
+
+
+def documentationNameForTraversal(traversal):
+  if traversal == 'Forward':
+    return 'collection'
+    if traversal == 'Bidirectional':
+      return 'bidirectional collection'
+    if traversal == 'RandomAccess':
+      return 'random-access collection'
+    assert False, 'unknown traversal'