[docs] Add GEP preconditions to UnsafePointer.

Offsets lower to GetElementPointer inbounds, which specifies that it's
Undefined Behaviour for the result to be out of bounds, regardless of if
the result is actually dereferenced.

Author: Alexis Beingessner

stdlib/public/core/UnsafePointer.swift.gyb

@@ -432,21 +432,29 @@ public struct ${Self}<Pointee>
   }
 
   /// Returns the next consecutive position.
+  ///
+  /// - Precondition: The result is within the bounds of the same allocation.
   public func successor() -> ${Self} {
     return self + 1
   }
 
   /// Returns the previous consecutive position.
+  ///
+  /// - Precondition: The result is within the bounds of the same allocation.
   public func predecessor() -> ${Self} {
     return self - 1
   }
 
   /// Returns `end - self`.
   public func distance(to x: ${Self}) -> Int {
     return x - self
+  ///
+  /// - Precondition: The result is within the bounds of the same allocation.
   }
 
   /// Returns `self + n`.
+  ///
+  /// - Precondition: The result is within the bounds of the same allocation.
   public func advanced(by n: Int) -> ${Self} {
     return self + n
   }
@@ -499,18 +507,22 @@ public func < <Pointee>(lhs: ${Self}<Pointee>, rhs: ${Self}<Pointee>) -> Bool {
 /// - Note: The following family of operator overloads are redundant
 ///   with Strideable. However, optimizer improvements are needed
 ///   before they can be removed without affecting performance.
+
+/// - Precondition: The result is within bounds of the same allocation.
 @_transparent
 public func + <Pointee>(lhs: ${Self}<Pointee>, rhs: Int) -> ${Self}<Pointee> {
   return ${Self}(Builtin.gep_Word(
     lhs._rawValue, rhs._builtinWordValue, Pointee.self))
 }
 
+/// - Precondition: The result is within the bounds of the same allocation.
 @_transparent
 public func + <Pointee>(lhs: Int,
            rhs: ${Self}<Pointee>) -> ${Self}<Pointee> {
   return rhs + lhs
 }
 
+/// - Precondition: The result is within the bounds of the same allocation.
 @_transparent
 public func - <Pointee>(lhs: ${Self}<Pointee>, rhs: Int) -> ${Self}<Pointee> {
   return lhs + -rhs
@@ -524,11 +536,13 @@ public func - <Pointee>(lhs: ${Self}<Pointee>, rhs: ${Self}<Pointee>) -> Int {
     / MemoryLayout<Pointee>.stride
 }
 
+/// - Precondition: The result is within the bounds of the same allocation.
 @_transparent
 public func += <Pointee>(lhs: inout ${Self}<Pointee>, rhs: Int) {
   lhs = lhs + rhs
 }
 
+/// - Precondition: The result is within the bounds of the same allocation.
 @_transparent
 public func -= <Pointee>(lhs: inout ${Self}<Pointee>, rhs: Int) {
   lhs = lhs - rhs