Clarify usage of `$errors` array within hook_civicrm_validateForm.md

docs/hooks/hook_civicrm_validateForm.md

@@ -30,7 +30,7 @@ This hook was introduced in v4.2
 -   $form - Reference to the civicrm form object. This is useful if you
     want to retrieve any values that we've constructed in the form
 -   $errors - Reference to the errors array. All errors will be added
-    to this array
+    to this array. Note that in using this array within the function, the key needs to be the name of the field next to which the error message should be displayed.
 
 ## Returns
 
@@ -93,4 +93,4 @@ In this case the form is 'Main' - in the Contribution Page flow.
 
 >     getAttribute('name');
 >
-> Probably works to get the name
\ No newline at end of file
+> Probably works to get the name