Add a better description to PossiblyInvalidArgument

Author: muglug

docs/running_psalm/issues/InvalidArgument.md

@@ -6,6 +6,35 @@ Emitted when a supplied function/method argument is incompatible with the method
 <?php
 
 class A {}
+
+function foo(A $a) : void {}
+
+/**
+ * @param string $s
+ */
+function callFoo($s) : void {
+    foo($s);
+}
+```
+
+## Why it’s bad
+
+Calling functions with incorrect values will cause a fatal error at runtime.
+
+## How to fix
+
+Sometimes this message can just be the result of an incorrect docblock.
+
+You can fix by correcting the docblock, or converting to a function signature:
+
+```php
+<?php
+
+class A {}
+
 function foo(A $a) : void {}
-foo("hello");
+
+function callFoo(A $a) : void {
+    foo($a);
+}
 ```