export to_sampleable and add to docs

Author: torfjelde

docs/src/api.md

@@ -20,6 +20,13 @@ One can nest models and call another model inside the model function with `left
 returned(::DynamicPPL.Model)
 ```
 
+Note that a `[returned(::DynamicPPL.Model)](@ref)` is only sampleable; one cannot compute `logpdf` for its realizations.
+This can be indicated using [`to_sampleable`](@ref) if the user wants to be explicit.
+
+```@docs
+to_sampleable
+```
+
 In the past, one would instead embed sub-models using [`@submodel`](@ref), which has been deprecated since the introduction of [`returned(model)`](@ref)
 
 ```@docs

src/DynamicPPL.jl

@@ -123,6 +123,7 @@ export AbstractVarInfo,
     unfix,
     prefix,
     returned,
+    to_sampleable,
     # Convenience macros
     @addlogprob!,
     @submodel,