[Doc] [SR-10094] Add missing documentation for SE-0237

https://bugs.swift.org/browse/SR-10094

Author: fassko

stdlib/public/core/Sequence.swift

@@ -363,6 +363,16 @@ public protocol Sequence {
     initializing ptr: UnsafeMutableBufferPointer<Element>
   ) -> (Iterator,UnsafeMutableBufferPointer<Element>.Index)
 
+  /// Call `body(p)`, where `p` is a pointer to the collection's
+  /// contiguous storage.  If no such storage exists, it is
+  /// first created.  If the collection does not support an internal
+  /// representation in a form of contiguous storage, `body` is not
+  /// called and `nil` is returned.
+  ///
+  /// A `Collection` that provides its own implementation of this method
+  /// must also guarantee that an equivalent buffer of its `SubSequence` 
+  /// can be generated by advancing the pointer by the distance to the
+  /// slice's `startIndex`.
   func withContiguousStorageIfAvailable<R>(
     _ body: (UnsafeBufferPointer<Element>) throws -> R
   ) rethrows -> R?