Add a note on the return codes

Author: avik-pal

docs/src/basics/NonlinearSolution.md

@@ -3,3 +3,14 @@
 ```@docs
 SciMLBase.NonlinearSolution
 ```
+
+## Return Code
+
+  - `ReturnCode.Success` - The nonlinear solve succeeded.
+  - `ReturnCode.ConvergenceFailure` - The nonlinear solve failed to converge due to stalling
+    or some limit of the solver was exceeded. For example, too many shrinks for trust
+    region methods, number of resets for Broyden, etc.
+  - `ReturnCode.Unstable` - This corresponds to
+    `NonlinearSafeTerminationReturnCode.ProtectiveTermination` and is caused if the step-size
+    of the solver was too large or the objective value became non-finite.
+  - `ReturnCode.MaxIters` - The maximum number of iterations was reached.