doc: Sync classref with apparent Variant return type changes

Part of those seem bogus, methods like Array.back()/front()
should return a Variant and not void.

doc/classes/PackedIntArray.xml

@@ -21,6 +21,8 @@
 			</description>
 		</method>
 		<method name="append">
+			<return type="void">
+			</return>
 			<argument index="0" name="integer" type="int">
 			</argument>
 			<description>
@@ -28,6 +30,8 @@
 			</description>
 		</method>
 		<method name="append_array">
+			<return type="void">
+			</return>
 			<argument index="0" name="array" type="PackedIntArray">
 			</argument>
 			<description>
@@ -53,11 +57,15 @@
 			</description>
 		</method>
 		<method name="invert">
+			<return type="void">
+			</return>
 			<description>
 				Reverses the order of the elements in the array.
 			</description>
 		</method>
 		<method name="push_back">
+			<return type="void">
+			</return>
 			<argument index="0" name="integer" type="int">
 			</argument>
 			<description>
@@ -65,6 +73,8 @@
 			</description>
 		</method>
 		<method name="remove">
+			<return type="void">
+			</return>
 			<argument index="0" name="idx" type="int">
 			</argument>
 			<description>
@@ -72,6 +82,8 @@
 			</description>
 		</method>
 		<method name="resize">
+			<return type="void">
+			</return>
 			<argument index="0" name="idx" type="int">
 			</argument>
 			<description>
@@ -79,6 +91,8 @@
 			</description>
 		</method>
 		<method name="set">
+			<return type="void">
+			</return>
 			<argument index="0" name="idx" type="int">
 			</argument>
 			<argument index="1" name="integer" type="int">