Clarified usage of TreeItem get_next*() and get_prev*() functions.

(cherry picked from commit df5a9228808497f19b5bddf7b58d2a532cd990d0)

doc/classes/TreeItem.xml

@@ -168,14 +168,14 @@
 		<method name="get_next">
 			<return type="TreeItem" />
 			<description>
-				Returns the next TreeItem in the tree or a null object if there is none.
+				Returns the next sibling TreeItem in the tree or a null object if there is none.
 			</description>
 		</method>
 		<method name="get_next_visible">
 			<return type="TreeItem" />
 			<argument index="0" name="wrap" type="bool" default="false" />
 			<description>
-				Returns the next visible TreeItem in the tree or a null object if there is none.
+				Returns the next visible sibling TreeItem in the tree or a null object if there is none.
 				If [code]wrap[/code] is enabled, the method will wrap around to the first visible element in the tree when called on the last visible element, otherwise it returns [code]null[/code].
 			</description>
 		</method>
@@ -188,14 +188,14 @@
 		<method name="get_prev">
 			<return type="TreeItem" />
 			<description>
-				Returns the previous TreeItem in the tree or a null object if there is none.
+				Returns the previous sibling TreeItem in the tree or a null object if there is none.
 			</description>
 		</method>
 		<method name="get_prev_visible">
 			<return type="TreeItem" />
 			<argument index="0" name="wrap" type="bool" default="false" />
 			<description>
-				Returns the previous visible TreeItem in the tree or a null object if there is none.
+				Returns the previous visible sibling TreeItem in the tree or a null object if there is none.
 				If [code]wrap[/code] is enabled, the method will wrap around to the last visible element in the tree when called on the first visible element, otherwise it returns [code]null[/code].
 			</description>
 		</method>