Update abstractsystem.jl

Author: ChrisRackauckas

src/systems/abstractsystem.jl

@@ -913,7 +913,12 @@ end
 """
 $(TYPEDSIGNATURES)
 
-Mark a system as completed. If a system is complete, the system will no longer
+Mark a system as completed. A completed system is a system which is done being
+defined/modified and is ready for structural analysis or other transformations.
+This allows for analyses and optimizations to be performed which require knowing
+the global structure of the system.
+        
+One property to note is that if a system is complete, the system will no longer
 namespace its subsystems or variables, i.e. `isequal(complete(sys).v.i, v.i)`.
 """
 function complete(sys::AbstractSystem; split = true)