doc: dt: Document the target root overlay method

Add a description of the target root overlay method to the overlay
documention file.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>

Documentation/devicetree/overlay-notes.txt

@@ -104,6 +104,10 @@ If your board has multiple slots/places where a single overlay can work
 and each slot is defined by a node, you can use the of_overlay_create_indirect()
 method to select the target.
 
+For overlays on probeable busses, use the of_overlay_create_target_root() method
+in which you supply a device node as a target root, and which all target
+references in the overlay are performed relative to that node.
+
 Overlay DTS Format
 ------------------
 
@@ -144,3 +148,7 @@ contains the information required to map from a phandle to a tree location.
 The indirect target requires the use of a selector target on the call to
 of_overlay_create_indirect(). I.e. passing the "foo" id will select the target
 in the foo node, "bar" in bar node, etc.
+
+Note that when using the target root create method all target references must
+lie under the target root node. I.e. the overlay is not allowed to 'break' out
+of the root.