new @register_symbolic example

Author: TorkelE

docs/src/model_creation/dsl_basics.md

@@ -265,12 +265,12 @@ Catalyst comes with the following predefined functions:
 - The activating/repressive Hill function: $hillar(X,Y,v,K,n) = v * (X^n)/(X^n + Y^n + K^n)$.
 
 ### [Registration of non-algebraic functions](@id dsl_description_nonconstant_rates_function_registration)
-Previously we showed how user-defined functions [can be used in rates directly](@ref dsl_description_nonconstant_rates_available_functions). For functions containing more complicated syntax (e.g. `for` loops or `if` statements), we must add an additional step: registering it using the `@register_symbolic` macro. Below we define a function which output depends on whether `X` is smaller or larger than a threshold value. Next, we register it using `@register_symbolic`, after which we can use it within the DSL.
+Previously we showed how user-defined functions [can be used in rates directly](@ref dsl_description_nonconstant_rates_available_functions). For functions containing more complicated syntax (e.g. `for` loops or `if` statements), we must add an additional step: registering it using the `@register_symbolic` macro. Below we define a non-standard function of one variable. Next, we register it using `@register_symbolic`, after which we can use it within the DSL.
 ```@example dsl_basics
-thresfunc(X) = (X < 10) ? X : 10.0
-@register_symbolic thresfunc(X)
+weirdfunc(x) = round(x) + 2.0
+@register_symbolic weirdfunc(X)
 rn = @reaction_network begin
-    thresfunc(X), 0 --> X
+    weirdfunc(X), 0 --> X
     d, X --> 0
 end
 ```