Add more docs

Author: muglug

docs/running_psalm/issues/DuplicateArrayKey.md

@@ -6,10 +6,10 @@ Emitted when an array has a key more than once
 <?php
 
 $arr = [
-    'a' => 1,
-    'b' => 2,
-    'c' => 3,
-    'c' => 4,
+    'a' => 'one',
+    'b' => 'two',
+    'c' => 'this text will be overwritten by the next line',
+    'c' => 'three',
 ];
 ```
 
@@ -21,9 +21,9 @@ Remove the offending duplicates:
 <?php
 
 $arr = [
-    'a' => 1,
-    'b' => 2,
-    'c' => 4,
+    'a' => 'one',
+    'b' => 'two',
+    'c' => 'three',
 ];
 ```
 

docs/running_psalm/issues/DuplicateClass.md

@@ -8,3 +8,25 @@ Emitted when a class is defined twice
 class A {}
 class A {}
 ```
+
+## Why this is bad
+
+The above code won’t compile.
+
+PHP does allow you to define a class conditionally:
+
+```php
+<?php
+
+if (rand(0, 1)) {
+    class A {
+        public function __construct(string $s) {}
+    }
+} else {
+    class A {
+        public function __construct(object $o) {}
+    }
+}
+```
+
+But Psalm _really_ doesn't want you to use this pattern – it's impossible for Psalm to know (without using reflection) which class is getting used.

docs/running_psalm/issues/DuplicateParam.md

@@ -7,3 +7,7 @@ Emitted when a function has a param defined twice
 
 function foo(int $b, string $b) {}
 ```
+
+## Why this is bad
+
+The above code produces a fatal error in PHP.