Crowdsale docs improvements. (#1446)

* Added a small Crowdsale fallback gas warning.

* Fixed a couple typos.

(cherry picked from commit 94692acb146b4204d7a57b604f160e46dba9c2cd)

contracts/crowdsale/Crowdsale.sol

@@ -73,6 +73,9 @@ contract Crowdsale {
 
   /**
    * @dev fallback function ***DO NOT OVERRIDE***
+   * Note that other contracts will transfer fund with a base gas stipend
+   * of 2300, which is not enough to call buyTokens. Consider calling
+   * buyTokens directly when purchasing tokens from a contract.
    */
   function () external payable {
     buyTokens(msg.sender);
@@ -100,7 +103,7 @@ contract Crowdsale {
   }
 
   /**
-   * @return the mount of wei raised.
+   * @return the amount of wei raised.
    */
   function weiRaised() public view returns (uint256) {
     return _weiRaised;
@@ -108,7 +111,7 @@ contract Crowdsale {
 
   /**
    * @dev low level token purchase ***DO NOT OVERRIDE***
-   * @param beneficiary Address performing the token purchase
+   * @param beneficiary Recipient of the token purchase
    */
   function buyTokens(address beneficiary) public payable {
 
@@ -188,7 +191,7 @@ contract Crowdsale {
   }
 
   /**
-   * @dev Executed when a purchase has been validated and is ready to be executed. Not necessarily emits/sends tokens.
+   * @dev Executed when a purchase has been validated and is ready to be executed. Doesn't necessarily emit/send tokens.
    * @param beneficiary Address receiving the tokens
    * @param tokenAmount Number of tokens to be purchased
    */