docs: add comment explaining added rrule

Co-authored-by: Dhairya Gandhi <[email protected]>

Author: AayushSabharwal

ext/SciMLBaseChainRulesCoreExt.jl

@@ -137,6 +137,10 @@ function ChainRulesCore.rrule(
     SciMLBase.IntervalNonlinearProblem(args...; kwargs...), IntervalNonlinearProblemAdjoint
 end
 
+# This is a workaround for the fact `NonlinearProblem` is a mutable struct. In SciMLSensitivity, we call
+# `back` explicitly while already in a reverse pass causing a nested gradient call. The mutable struct
+# causes accumulation anytime `getfield/property` is called, accumulating multiple times. This tries to treat
+# AbstractDEProblem as immutable for the purposes of reverse mode AD.
 function ChainRulesCore.rrule(::ChainRulesCore.RuleConfig{>:ChainRulesCore.HasReverseMode}, ::typeof(Base.getproperty), x::NonlinearProblem, f::Symbol)
     val = getfield(x, f)
     function back(der)