Add docs

Author: YingboMa

src/systems/abstractsystem.jl

@@ -846,9 +846,23 @@ end
 
 iscomplete(sys::AbstractSystem) = isdefined(sys, :complete) && getfield(sys, :complete)
 
+"""
+$(TYPEDSIGNATURES)
+
+Mark a system as scheduled. It is only intended in compiler internals. A system
+is scheduled after tearing based simplifications where equations are converted
+into assignments.
+"""
 function schedule(sys::AbstractSystem)
     has_schedule(sys) ? sys : (@set! sys.isscheduled = true)
 end
+
+"""
+$(TYPEDSIGNATURES)
+
+If a system is scheduled, then changing its equations, variables, and
+parameters is no longer legal.
+"""
 function isscheduled(sys::AbstractSystem)
     if has_schedule(sys)
         get_schedule(sys) !== nothing