Update documentation

Author: Frederick Kellison-Linn

Foundation/NSData.swift

@@ -945,6 +945,17 @@ open class NSMutableData : NSData {
 
     }
 
+    /// Replaces the bytes in `range` with `replacementLength` bytes
+    /// beginning from the location pointed to by `replacementBytes`.
+    ///
+    /// - Parameter range: range of bytes in `self` to be replaced.
+    /// - Parameter replacementBytes: pointer to beginning of buffer
+    ///   where replacement bytes can be found.
+    /// - Parameter replacementLength: the number of bytes to copy from
+    ///   `replacementBytes`.
+    ///
+    /// - Note: `replacementLength` must be less than or equal to the
+    ///   size of the buffer pointed to by `replacementBytes`. 
     open func replaceBytes(in range: NSRange, withBytes replacementBytes: UnsafeRawPointer?, length replacementLength: Int) {
         let bytePtr = replacementBytes?.bindMemory(to: UInt8.self, capacity: replacementLength)
         CFDataReplaceBytes(_cfMutableObject, CFRangeMake(range.location, range.length), bytePtr, replacementLength)