Mention that Array.front/back throw error if empty

doc/classes/Array.xml

@@ -119,7 +119,7 @@
 			<return type="Variant">
 			</return>
 			<description>
-				Returns the last element of the array, or [code]null[/code] if the array is empty.
+				Returns the last element of the array. Throws an error and returns [code]null[/code] if the array is empty.
 			</description>
 		</method>
 		<method name="bsearch">
@@ -216,7 +216,7 @@
 			<return type="Variant">
 			</return>
 			<description>
-				Returns the first element of the array, or [code]null[/code] if the array is empty.
+				Returns the first element of the array. Throws an error and returns [code]null[/code] if the array is empty.
 			</description>
 		</method>
 		<method name="has">