Update functionCall docs

(cherry picked from commit 242400e)

Author: nventuro

contracts/utils/Address.sol

@@ -66,6 +66,9 @@ library Address {
      * If `target` reverts with a revert reason, it is bubbled up by this
      * function (like regular Solidity function calls).
      *
+     * Returns the raw returned data. To convert to the expected return value,
+     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
+     *
      * Requirements:
      *
      * - `target` must be a contract.
@@ -88,18 +91,13 @@ library Address {
     }
 
     /**
-     * @dev Performs a Solidity function call using a low level `call`,
-     * transferring `value` wei. A plain`call` is an unsafe replacement for a
-     * function call: use this function instead.
-     *
-     * If `target` reverts with a revert reason, it is bubbled up by this
-     * function (like regular Solidity function calls).
+     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
+     * but also transferring `value` wei to `target`.
      *
      * Requirements:
      *
-     * - `target` must be a contract.
      * - the calling contract must have an ETH balance of at least `value`.
-     * - calling `target` with `data` must not revert.
+     * - the called Solidity function must be `payable`.
      *
      * _Available since v3.1._
      */