[Documentation] set error placement in contract, match example to prescription

Add example

fix order

Author: fulldecent

docs/style-guide.rst

@@ -1058,14 +1058,40 @@ Inside each contract, library or interface, use the following order:
 1. Type declarations
 2. State variables
 3. Events
-4. Modifiers
-5. Functions
+4. Errors
+5. Modifiers
+6. Functions
 
 .. note::
 
     It might be clearer to declare types close to their use in events or state
     variables.
 
+Yes:
+
+.. code-block:: solidity
+
+    // SPDX-License-Identifier: GPL-3.0
+    pragma solidity >=0.8.4 <0.9.0;
+
+    abstract contract Math {
+        error DivideByZero();
+        function divide(int256 numerator, int256 denominator) public virtual returns (uint256);
+    }
+
+No:
+
+.. code-block:: solidity
+
+    // SPDX-License-Identifier: GPL-3.0
+    pragma solidity >=0.8.4 <0.9.0;
+
+    abstract contract Math {
+        function divide(int256 numerator, int256 denominator) public virtual returns (uint256);
+        error DivideByZero();
+    }
+
+
 ******************
 Naming Conventions
 ******************
@@ -1130,15 +1156,15 @@ Yes:
     contract Owned {
         address public owner;
 
-        constructor() {
-            owner = msg.sender;
-        }
-
         modifier onlyOwner {
             require(msg.sender == owner);
             _;
         }
 
+        constructor() {
+            owner = msg.sender;
+        }
+
         function transferOwnership(address newOwner) public onlyOwner {
             owner = newOwner;
         }
@@ -1169,15 +1195,15 @@ No:
     contract owned {
         address public owner;
 
-        constructor() {
-            owner = msg.sender;
-        }
-
         modifier onlyOwner {
             require(msg.sender == owner);
             _;
         }
 
+        constructor() {
+            owner = msg.sender;
+        }
+
         function transferOwnership(address newOwner) public onlyOwner {
             owner = newOwner;
         }