Add note about constraints in documentation

docs/backend.md

@@ -20,6 +20,7 @@ git clone https://github.com/MilesCranmer/SymbolicRegression.jl
     - Parts of the code which are typically edited by users include:
         - [`src/LossFunctions.jl`](https://github.com/MilesCranmer/SymbolicRegression.jl/blob/master/src/LossFunctions.jl), particularly the function `eval_loss`. This function assigns a loss to a given expression, using `eval_tree_array` to evaluate it, and `loss` to compute the loss with respect to the dataset.
         - [`src/CheckConstraints.jl`](https://github.com/MilesCranmer/SymbolicRegression.jl/blob/master/src/CheckConstraints.jl), particularly the function `check_constraints`. This function checks whether a given expression satisfies constraints, such as having a complexity lower than `maxsize`, and whether it contains any forbidden nestings of functions.
+            - Note that all expressions, *even intermediate expressions*, must comply with constraints. Therefore, make sure that evolution can still reach your desired expression (with one mutation at a time), before setting a hard constraint. In other cases you might want to instead put in the loss function.
         - [`src/Options.jl`](https://github.com/MilesCranmer/SymbolicRegression.jl/blob/master/src/Options.jl), as well as the struct definition in [`src/OptionsStruct.jl`](https://github.com/MilesCranmer/SymbolicRegression.jl/blob/master/src/OptionsStruct.jl). This file specifies all the options used in the search: an instance of `Options` is typically available throughout every function in `SymbolicRegression.jl`. If you add new functionality to the backend, and wish to make it parameterizable (including from PySR), you should specify it in the options.
         - For reference, the main loop itself is found in the `EquationSearch` function inside [`src/SymbolicRegression.jl`](https://github.com/MilesCranmer/SymbolicRegression.jl/blob/master/src/SymbolicRegression.jl).
 3. Specify the directory of `SymbolicRegression.jl` to PySR by setting `julia_project` in the `PySRRegressor` object, and run `.fit` when you're ready. That's it! No compilation or build steps required.