Improve SignedSafeMath docs

contracts/math/SignedSafeMath.sol

@@ -9,8 +9,15 @@ pragma solidity ^0.6.0;
 library SignedSafeMath {
     int256 constant private _INT256_MIN = -2**255;
 
-    /**
+        /**
-     * @dev Multiplies two signed integers, reverts on overflow.
+     * @dev Returns the multiplication of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `*` operator.
+     *
+     * Requirements:
+     *
+     * - Multiplication cannot overflow.
      */
     function mul(int256 a, int256 b) internal pure returns (int256) {
         // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
@@ -29,7 +36,16 @@ library SignedSafeMath {
     }
 
     /**
-     * @dev Integer division of two signed integers truncating the quotient, reverts on division by zero.
+     * @dev Returns the integer division of two signed integers. Reverts on
+     * division by zero. The result is rounded towards zero.
+     *
+     * Counterpart to Solidity's `/` operator. Note: this function uses a
+     * `revert` opcode (which leaves remaining gas untouched) while Solidity
+     * uses an invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     *
+     * - The divisor cannot be zero.
      */
     function div(int256 a, int256 b) internal pure returns (int256) {
         require(b != 0, "SignedSafeMath: division by zero");
@@ -41,7 +57,14 @@ library SignedSafeMath {
     }
 
     /**
-     * @dev Subtracts two signed integers, reverts on overflow.
+     * @dev Returns the subtraction of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `-` operator.
+     *
+     * Requirements:
+     *
+     * - Subtraction cannot overflow.
      */
     function sub(int256 a, int256 b) internal pure returns (int256) {
         int256 c = a - b;
@@ -51,7 +74,14 @@ library SignedSafeMath {
     }
 
     /**
-     * @dev Adds two signed integers, reverts on overflow.
+     * @dev Returns the addition of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `+` operator.
+     *
+     * Requirements:
+     *
+     * - Addition cannot overflow.
      */
     function add(int256 a, int256 b) internal pure returns (int256) {
         int256 c = a + b;