Improve ERC20.decimals documentation (#3933)

Author: frangio

contracts/token/ERC20/ERC20.sol

@@ -18,6 +18,9 @@ import "../../utils/Context.sol";
  * https://forum.openzeppelin.com/t/how-to-implement-erc20-supply-mechanisms/226[How
  * to implement supply mechanisms].
  *
+ * The default value of {decimals} is 18. To change this, you should override
+ * this function so it returns a different value.
+ *
  * We have followed general OpenZeppelin Contracts guidelines: functions revert
  * instead returning `false` on failure. This behavior is nonetheless
  * conventional and does not conflict with the expectations of ERC20
@@ -45,9 +48,6 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
     /**
      * @dev Sets the values for {name} and {symbol}.
      *
-     * The default value of {decimals} is 18. To select a different value for
-     * {decimals} you should overload it.
-     *
      * All two of these values are immutable: they can only be set once during
      * construction.
      */
@@ -77,8 +77,8 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
      * be displayed to a user as `5.05` (`505 / 10 ** 2`).
      *
      * Tokens usually opt for a value of 18, imitating the relationship between
-     * Ether and Wei. This is the value {ERC20} uses, unless this function is
-     * overridden;
+     * Ether and Wei. This is the default value returned by this function, unless
+     * it's overridden.
      *
      * NOTE: This information is only used for _display_ purposes: it in
      * no way affects any of the arithmetic of the contract, including