Add minimal documentation to the num class.

lib/core/num.dart

Index: lib/core/num.dart
diff --git a/lib/core/num.dart b/lib/core/num.dart
index f8d5842f8856d4c60e3e74e9c9ab1944fc7339ad..3036560255f264b3dcf7431b45f753e70c98c926 100644
--- a/lib/core/num.dart
+++ b/lib/core/num.dart
@@ -5,40 +5,104 @@
 // Dart core library.
 
 abstract class num implements Comparable, Hashable {
-  // Arithmetic operations.
+  /** Addition operator. */
   num operator +(num other);
+
+  /** Subtraction operator. */
   num operator -(num other);
+
+  /** Multiplication operator. */
   num operator *(num other);
+
+  /** Euclidean modulo operator. */
   num operator %(num other);
+
+  /** Division operator. */
   double operator /(num other);
-  // Truncating division.
+
+  /**
+   * Truncating division operator.
+   *
+   * The result of the truncating division [:a ~/ b:] is equivalent to
+   * [:(a / b).truncate():].
+   */
   num operator ~/(num other);
-  // The unary '-' operator.
+
+  /** Negate operator. */
   num operator -();
+
+  /** Return the remainder from dividing this [num] by [other]. */
   num remainder(num other);
 
-  // Relational operations.
+  /** Relational less than operator. */
   bool operator <(num other);
+
+  /** Relational less than or equal operator. */
   bool operator <=(num other);
+
+  /** Relational greater than operator. */
   bool operator >(num other);
+
+  /** Relational greater than or equal operator. */
   bool operator >=(num other);
 
-  // Predicates.
   bool isNaN();
+
   bool isNegative();
+
   bool isInfinite();
 
+  /** Returns the absolute value of this [num]. */
   num abs();
-  num round();
+
+  /** Returns the greatest integer value no greater than this [num]. */
   num floor();
+
+  /** Returns the least integer value that is no smaller than this [num]. */
   num ceil();
+
+  /**
+   * Returns the integer value closest to this [num].
+   *
+   * Rounds away from zero when there is no closest integer:
+   *  [:(3.5).round() == 4:] and [:(-3.5).round() == -4:].
+   */
+  num round();
+
+  /**
+   * Returns the integer value obtained by discarding any fractional
+   * digits from this [num].
+   */
   num truncate();
 
+  /** Truncates this [num] to an integer and returns the result as an [int]. */
   int toInt();
+
+  /**
+   * Return this [num] as a [double].
+   *
+   * If the number is not representable as a [double], an
+   * approximation is returned. For numerically large integers, the
+   * approximation may be infinite.
+   */
   double toDouble();
 
+  /**
+   * Converts a [num] to a string representation with [fractionDigits]
+   * digits after the decimal point.
+   */
   String toStringAsFixed(int fractionDigits);
+
+  /**
+   * Converts a [num] to a string in decimal exponential notation with
+   * [fractionDigits] digits after the decimal point.
+   */
   String toStringAsExponential(int fractionDigits);
+
+  /**
+   * Converts a [num] to a string representation with [precision]
+   * significant digits.
+   */
   String toStringAsPrecision(int precision);
 
   /**