Update gnmi-specification.md

Added explicit deletion verbiage for TARGET-DEFINED.

rpc/gnmi/gnmi-specification.md

@@ -1707,6 +1707,10 @@ branch that applies to a multitude of leaves, e.g. when removing a logical inter
 in a configuration, deletes could be issued at container level branches that apply to
 that interface, rather than all the individual leaves.
 
+Explicit deletion also applies to TARGET-DEFINED subscription mode.  
+Whereas, if the TARGET-DEFINED subscription determines the best type of delivery to be ON-CHANGE, 
+explicit deletion is required and if decided to be SAMPLE, deletion will be optional.
+
 To replace the contents of an entire node within the tree, the target populates
 the `delete` field with the path of the node being removed, along with the new
 contents within the `update` field.