Document floating point descriptions.

Fixes <rdar://problem/51409021>.

Author: amartini51

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -92,6 +92,12 @@ public struct ${Self} {
 ${Availability(bits)}
 extension ${Self}: CustomStringConvertible {
   /// A textual representation of the value.
+  ///
+  /// For all  finite values, the value of this property is a string that can
+  /// be converted back to an instance of ${Self} without rounding errors.
+  /// That is, if `x` in a `${Self}`, then `${Self}(x.description) == x` is
+  /// always true.  For any NaN value, the property's value is "nan", and for
+  /// positive and negative infinity its value is "inf" and "-inf".
   public var description: String {
     if isNaN {
       return "nan"
@@ -108,6 +114,9 @@ extension ${Self}: CustomStringConvertible {
 ${Availability(bits)}
 extension ${Self}: CustomDebugStringConvertible {
   /// A textual representation of the value, suitable for debugging.
+  ///
+  /// This property has the same value as the `description` propyrty, except
+  /// that NaN values are printed in an extended format.
   public var debugDescription: String {
     var (buffer, length) = _float${bits}ToString(self, debug: true)
     return buffer.withBytes { (bufferPtr) in